Get super gravity gun in any level
Enable the "map d3_citadel_03.bsp" code. Once you get the super gravity gun, enable the "changelevel [map name]" code to go to the desired level. Then, enable the "give weapon_physcannon" code.

Defend yourself by using the Zero-Point Energy gun
Take something like a barrel, chair, or another object with the Zero-Point Energy gun. Enemies cannot hurt you when they are shooting you. For better results, crouch when you are defending yourself.

Avoid losing health in Sand Trap
When you start the Sands Trap map, you will find some rocks. Someone will tell you not to step on the sand because a creature will attack. You will have to jump on the rocks. When the rock ends and only sand remains, use your gravity gun. Use it to place a door or wooden piece in the sand and stand on it. Then, place another object in front of it. Jump on it, remove the first object, and place it in front. Continue to do this until you reach rock again and the creature will not appear.

Easy kills
To kill an enemy quickly and easily, take the shotgun and hit an enemy like headcrap, zombies, and City 17 guards. Aim and hit it in the head to kill it. Additionally, you can use Dog's ball to kill; or distract and attract enemies to the ball for easy kills. When you first get the anti-gravity gun while you play fetch with "Dog", make sure to keep the ball. The ball is actually a "rollermine". You can keep the ball up until you go over to Dr. Kliener's fortified graveyard. The rollermine (ball) will follow you almost everywhere and is harmless to you, but it actually kills both types of Head Crabs. Other enemies such as zombies will ignore you and swing violently at the ball, giving you plenty of time to kill them, even with the crowbar. This will save you lots of ammunition throughout the zombie levels. You must be careful not to explode the ball, or it will be gone. If this happens, just re-load from where you had it last. The ball will follow you up most stairs, but you must carry it with the gravity gun up ladders.

Private torture
On the first level when the combine guard (who turns out to be Barney in disguise) tells you to follow him, do as he says. While you are walking down the corridor, look through the small gap in the first door you come across. You will see a man sitting in a chair, that has blood splattered on the floor underneath it that says, "There has to be some sort of mistake. I got a standard relocation coupon juts like everybody else." A combine guard who is in there with him will then walk up to the door and close the shutter.

Crazy lab
The "all-knowing-Vortiguant" can be found in another location. Enable the map "d1_eli_01" code. After the map loads, find your way inside the airlock. After the scanning sequence finishes, you should see the right doors opening. Directly in front of you will be a fence, and beyond it a blue pick-up. Ignore Dr. Mossman, who is talking to you, and enable the "noclip" code. Go through the truck and you will see a man in a blue shirt with no eyes. Ignore him and go down the ramp down. On the left side of the large space will be a Vortigaunt. Disable the "noclip" code and talk to him as many times as you can to see that he is crazy.

Bonus sequence
After the credits, wait to see the headcrab, Lamar, being pursued by the scientist. Then, watch him pounce on your head. Additionally, after you complete the game, watch the credits. Note: They cannot be skipped. When you reach the end of the credits, you will see Lamar jump onscreen. You will then hear Dr. Kliener say, "Lamar? Blast that little… Where did she get to?" This is the same thing that he says when you first see him looking for her. Lamar will then lash out at you and supposedly "couple with your head".

Defeating Barnicals
To kill the Barnicals, let them pull you up, than hit them with a crowbar from a short distance.

Defeating Striders
If you find it difficult to hit the Striders (or any other enemy) with the RPG from far away, fire a rocket and immediately press [Zoom] (default is Z). You will automatically zoom in like normal, but will still have control of the rocket. This helps in the later levels when you do not want to get too close to the Striders, or other enemies like the Combines.

Blazing Saddles reference
When you teleport to Dr. Kliener's lab from Nova Prospekt, you will meet up with Dog and go out and find Barney. Do not follow Dog out of the lab yet; wait until Dr. Kliener and Alyx start talking about getting another headcrab to replace Lamar (Dr. Kliener's pet headcrab). After Alyx suggests that he gets another headcrab, he replies, "There is only one heady." In the movie Blazing Saddles, the villain's name is Headly Lamar (whom is called Heady Lamar by some of the characters).

Half-Life logo reference
Occasionally you will see the Half-Life Logo spray painted on a wall. One example is on the wall next the entrance to Dr. Kliener's lab.

Lamar passes through crates
When Barney turns on the lights to the room that your Hazard Suit is in at Dr. Kliener's lab, he finds Lamar (Dr. Kliener's pet Head Crab). She (Lamar) attacks Barney. Barney then throws her onto the floor and she slams into some crates. Notice that she passes through them.

Invisible Lamar
When you first meet Lamar (Dr. Kliener's pet Head Crab), enable the impulse 101 code and shoot her with any gun. Lamar will instantly disappear. You will see some blood on the floor after she disappears if you shoot her with your pistol. Alyx, Barney, and Dr. Kliener will all act as if she is still there. Only Barney and Dr. Kliener do not look on top of the cabinet where she is supposed to be. They will look in completely different directions. Lamar will later return in the Teleporter room. You can shoot her again and the same thing will happen.

Lamar passes through Dr. Kliener's leg
When you and Alyx go to Dr. Kliener's lab via the Teleporter, Alyx bangs on the door and asks Dr. Kliener to let you out. Dr. Kliener then opens the door holding a shotgun. You will then see Lamar appear at Dr. Kliener's feet. However, she passes through Dr. Kliener's right leg in the process.

Jumpy Dr. Kliener and shotgun; invincible and vanishing Lamar
When you and Alyx go to Dr. Kliener's lab via the Teleporter, enable the noclip code and go through the door into Dr. Kliener's lab. You will see Dr. Kliener and Lamar behind the door; both of them not moving and a shotgun lying of the floor next to Dr. Kliener. When the moment arrives for Dr. Kliener to open the door, he suddenly jumps towards the door to open it, and the shotgun suddenly jumps into his hands. If you try to shoot Lamar, she will not die. After Lamar runs out of Alyx's view, she just disappears around the corner (and is not actually hiding at all).

Frozen Hawaiian hula doll
When you first enter Dr. Kliener's lab, you will find a Hawaiian hula doll on a desk with some monitors on it. If you bash into the desk, you will make it dance. However, when you and Alyx enter Dr. Kliener's lab again via the Teleporter and try to make it dance again, it will not. It is completely frozen on the spot.

Dog spawns from nowhere
When you and Alyx go back to Dr. Kliener's lab via the teleporter, Dr. Kliener shows Alyx that Dog has arrived. If you stand at the very back of the room (in front of the green door) and crouch, you can see underneath the door to the room where Dog is. However, he is not there. Just as Dr. Kliener opens the door, Dog suddenly spawns out of nowhere in the room, just in time for him to run out.

Pass through Dog's arms
After you and Dog leave Dr. Kliener's lab (after the Blazing Saddles reference), Dog waits for you to jump down a level. While Dog waits for you, he is completely still. He will not walk away from you when you get close to him. If you approach his arms, you can pass through them.

Dog's wires pass through Alyx
When Dr. Kliener tells Alyx that Dog arrived at his lab several days ago and lets him out, Dog rushes out and hugs Alyx. However, you can see that one of Dog's wires attached to his arms will pass through Alyx.

Door to nothingness
After you and Dog leave Dr. Kliener's lab (after the Blazing Saddles reference), enable the noclip code and go through the drink machines blocking you from going back to the beginning of the game. Go up to the door ahead of you. Disable the noclip code and open the door. You will see that there is nothing behind it, and you can just walk into nothingness.

Backless clock
When you and your squad are inside that big building and trying to disable the "generators", you will eventually encounter a clock that is hanging on a wall above an archway. To find it, enter the building and go up the stairs to the right. Go underneath the archway ahead of you, turn around, then look up to find it. Equip your Gravity Gun and shoot it using Mouse 1. Keep shooting it until it is on the floor, with its face and hands facing the floor (so you cannot see them). Notice that it has no back. You will be able to see through it. You may lose it -- to find it, crouch on the floor and look for a semi-circular shape and shoot it with your Gravity Gun until you can see its face and hands again.

Cheat Codes
Open Steam, go to your "Play Games" menu, and select Half-Life 2. Right click on it and select "Properties". Click the Launch Options button, then type "-console". Click "Ok", then close. Double click Half-Life 2 to start the game. Once it is loaded and you are playing, press the ~ key to bring up the console. Then, enter "sv_cheats 1" and press [Enter]. You can now enter the following codes at the console window. Also, any code can be used as a command line parameter by prefixing it with the "-" character. Note: For games that were downloaded from Steam, to use cheats in single player mode, start the game with the "-applaunch -dev -tconsole" command line parameter.

Result    Cheat Code
God mode (server only) god
Walk through objects (server only) noclip
See through walls mat_depthbias_normal 1
All weapons impulse 101
Spawn a jeep impulse 82
Spawn an air boat impulse 83
Ignored by NPCs notarget
Load indicated map map [map name]
Spawn indicated item give [item name]
Reduce your health buddha
Damage player hurtme [amount]
List maps maps
Disable dead bodies going through each other cl_ragdoll_collide 1
Toggle developer mode; 2 is verbose developer [0-2]
Toggle picker mode picker
Toggle HUD display cl_drawhud [0 or 1]
Toggle HUD display cl_enablehud [0 or 1]
Toggle frame rate display cl_showfps [0 or 1]
Enable mouse look +mlook
Show crosshairs hud_quickhelp/text? 1
Sets the size of carried weapon; 54 is default viewmodel_fov [number]
Execute script file exec [filename]
Creates an NPC; only where NPC are allowed npc_create [name]
Create an NPC aiming away from player npc_create_aimed
Move player to indicated location; requires
sv_cheats 1 setpos [coordinates]
Set view to indicated pitch yaw; requires
sv_cheats 1 Setang [value]
Set gravity value sv_gravity [number]
Set minimum stopping speed when on ground sv_stopspeed [number]
Set world friction sv_friction [number]
Set bounce multiplier for physically simulated
objects collision sv_bounce [number]
Set maximum velocity of any moving object sv_maxvelocity [number]
Set vertical view fixup when eyes are near water
plane sv_waterdist [number]
Slow time; 1 is default cl_phys_timescale [0.00-1.00]
Change air density air_density [number]
Set length of explosion confusion dsp_explosion_effect_duration [number]
Toggle bounding-boxes debug mode; red: ignore
damage, white: respond to damage, green: health prop_debug
Toggle reporting missing .WAV files sv_soundemitter_filecheck
Limits the number of texture units; 0 lets the game
decide mat_numtextureunits [number]
Move player to indicated location setpos [coordinates]
Set view to indicated pitch yaw Setang [value]
Wire frame models mat_wireframe1
Only wireframe objects effected by physics vcollide_wireframe 1
Toggle bump mapping mat_fastnobump [0 or 1]
No weapon model onscreen impulse 200
Deleted targeted object or person impulse 203
Show game triggers showtriggers_toggle
Show normal maps mat_normalmaps 1
Show surface materials mat_normals 1
Black and white screen mat_yuv 1
Color screen mat_yuv 0

Map names
Use one of the following values with the "map [map name]" code:

d1_canals_01
d1_canals_01a
d1_canals_02
d1_canals_03
d1_canals_05
d1_canals_06
d1_canals_07
d1_canals_08
d1_canals_09
d1_canals_10
d1_canals_11
d1_canals_12
d1_canals_13
d1_eli_01
d1_eli_02
d1_town_01
d1_town_01a
d1_town_02
d1_town_02a
d1_town_03
d1_town_04
d1_town_05
d1_trainstation_01
d1_trainstation_02
d1_trainstation_03
d1_trainstation_04
d1_trainstation_05
d1_trainstation_06
d2_coast_01
d2_coast_03
d2_coast_04
d2_coast_05
d2_coast_07
d2_coast_08
d2_coast_09
d2_coast_10
d2_coast_11
d2_coast_12
d2_prison_01
d2_prison_02
d2_prison_03
d2_prison_04
d2_prison_05
d2_prison_06
d2_prison_07
d2_prison_08
d3_breen_01
d3_c17_01
d3_c17_02
d3_c17_03
d3_c17_04
d3_c17_05
d3_c17_06a
d3_c17_06b
d3_c17_07
d3_c17_08
d3_c17_09
d3_c17_10a
d3_c17_10b
d3_c17_11
d3_c17_12
d3_c17_12b
d3_c17_13
d3_citadel_01
d3_citadel_02
d3_citadel_03
d3_citadel_04
d3_citadel_05

Weapon names
Use one of the following values with the "give [item name]" code:

weapon_357 (.357 Magnum)
weapon_alyxgun (may not work at certain times, and may cause undesired effects)
weapon_ar2 (pulse gun)
weapon_bugbait (Pheropod)
weapon_cguard (may not work at certain times, and may cause undesired effects)
weapon_crowbar (crowbar)
weapon_crossbow (crossbow)
weapon_frag (grenade)
weapon_physcannon (Gravity gun; may not work at certain times, and may cause undesired effects)
weapon_pistol (9mm pistol)
weapon_rpg (RPG)
weapon_shotgun (shotgun)
weapon_smg1 (submachine gun)
weapon_cubemap (may not work at certain times, and may cause undesired effects)
weapon_annabelle (Father Annabelle Grigori's shotgun, uses 357 magnum ammo)

Item names
Use one of the following values with the "give [item name]" code:

item_ar2_grenade (submachine gun grenades)
item_ammo_smg1_grenade (submachine gun grenades)
item_box_buckshot (shotgun ammo)
item_box_lrounds (pulse gun ammo)
item_ammo_ar2 (pulse gun ammo)
item_ammo_ar2_large (pulse gun ammo large)
item_large_box_lrounds (pulse gun ammo large)
item_ammo_ar2_altfire (pulse gun grenades)
item_box_mrounds (submachine gun ammo)
item_ammo_smg1_large (submachine gun ammo large)
item_large_box_mrounds (submachine gun ammo large)
item_ammo_smg1 (submachine gun ammo)
item_box_srounds (9mm pistol ammo)
item_ammo_pistol (9mm pistol ammo)
item_ammo_pistol_large (9mm pistol ammo large)
item_large_box_srounds (9mm pistol ammo large)
item_healthkit (large health kit; 25 HP)
item_battery (HEV suit battery; 15 AP)
item_ml_grenade (RPG ammo)
item_rpg_round (RPG ammo)
item_suit (HEV suit)
item_healthvial (small health kit; 10 HP)
item_ammo_357 (.357 Magnum ammo)
item_ammo_357_large (.357 Magnum ammo large )
item_ammo_crossbow (crossbow ammo)

NPC names
Use one of the following values with the "npc_create [name]" code:

npc_alyx
npc_antlion
npc_antlionguard
npc_barnacle
npc_barney
npc_breen
npc_citizen
npc_combine_s
npc_combinedropship
npc_combinegunship
npc_crow
npc_cscanner
npc_dog
npc_eli
npc_fastzombie
npc_gman
npc_headcrab
npc_headcrab_black
npc_headcrab_fast
npc_headcrab_poison
npc_helicopter
npc_ichthyosaur
npc_kleiner
npc_launcher
npc_manhack
npc_metropolice
npc_monk
npc_mossman
npc_pigeon
npc_poisonzombie
npc_rollermine
npc_seagull
npc_stalker
npc_strider
npc_turret_ceiling (ceiling turret)
npc_turret_floor (combine turret)
npc_vortigaunt
npc_zombie
npc_zombie_torso

Additional command-line parameters
Start the game with one of the following command line parameters to activate the corresponding feature:

Result    Command Line Parameter
Full screen mode -fullscreen
Set heap size -heapsize 80000
Set window size -width 1024 -height 768
Graph connection throughput in multi-player; 0 is off, 3 is most detailed -netgraph [0-3]
Force DirectX <version> compatibility; less than 8 not advised -dxlevel [number]0
Load all level data at start of level -preload
Use indicated filename for shader library -shaderdll [filename]
Look around by using mouse -mouselook
Add debug messages to ingame console -condebug
Software mode for rendering -sw
Toggle anamorphic widescreen mode -r_anamorphic [0 or 1]
Run in window -startwindowed
Run in window -window
Run in window -windowed
Unknown -mat_preloadshaders

Hidden G-Man
After you finally get out of Ravenholm, you will go through a tunnel-type area with tons of Headcrabs, past the mine cart with the big spinning deathtrap, and finally see a light at the end of the tunnel. You will exit into a train station. Normally, you would go all the way down until some crashed trains block you from going further, forcing you to go left into a fenced area full of combine. Enable the "noclip" code and walk through all the crashed trains. You come out in an open area with tracks. In the middle of the tracks is G-Man with his briefcase. Regrettably, you cannot talk to him or do anything else in this area.

G-Man sightings
G-Man keeps an omnipresent watch over your movements throughout your adventure in and around City 17. His locations are as follows:

1. Chapter 1: At the beginning.

2. Chapter 2: On a monitor screen in Dr. Kleiner's lab after Barney finishes at the terminal.

3. Chapter 3: On the TV that the Vortigaunt is watching inside the red train carriage.

4. Chapter 4: Standing on the pier in front of Station 7 at the beginning of the chapter.

5. Chapter 4: Flickering on a giant monitor attached to the high-rise building. It is very quick, but if you look carefully you can see the skeletal form of G-Man on the screen.

6. Chapter 6: On a small gantry near the second covered bridge.

7. Chapter 6: In the last part of the chapter you will come across a pier next to a dam. Get onto the pier by driving up the ramp in your speedboat and turn left by the big red warehouse. You can see G-Man staring to the right at the bottom of the rest of the pier.

8. Walking into a train tunnel beyond the railcar blocking the railroad. See the "Hidden G-Man" hint for a detailed description. Note: This sighting is not in the official strategy guide.

9. Chapter 9: Looking through a window near a pair of double doors on a monitor screen at the beginning.

10. Chapter 11: On a TV at the ruined foyer at the tenement block after you and Dog get separated.

11. Chapter 14: At the end.

Being disruptive
When you enter Dr. Kliener's lab, there is a large amount of items that you can just pick up and toss around: books, boxes, computer monitors, a mini cactus, and Dr. Kliener's clip board with the information for your Hazard Suit on it. If you toss things around the lab for long enough, Dr. Kliener, Barney or Alyx will say something to you:

Dr. Kliener: "Oh, do be careful."; "Careful."; and "Oh dear."

Barney: "Whoa, careful." and "You're pushing it, Gordon."

Alyx: "Careful." and "Take it easy, Gordon. "

Note: It will take a lot of attempts to make Alyx, Barney or Dr. Kliener say something -- be patient.

Strange squad comments
When you are in City 17 during your second visit, try talking to some of your squad members. They will say something like, "I can't believe this day has finally come", or "If I ever get my hands on Dr. Breen...". However, sometimes they will say strange and sometimes funny things like, "When this is all over, I'm gonna mate.", "Have I ever told you that I sometimes dream of cheese?", and more. Some of your squad members reply to what they have said. If you talk to a squad member that is standing near or next to another squad member, they will interact with each other. For example, If one of them says, "If I ever get my hands on Dr. Breen....", the other will reply, "That's more information that I require." The following is a list of all the things they say and all the replies. Note: The replies are not always the same, but they are most of the time.

I don't feel anything anymore.
You talking to me?
I can't remember the last time I had a shower.
Don't be so sure of that.
I don't think this war's ever going to end.
You're talking to yourself again.
To think, I all I wanted to do was to sell insurance.
I'm with you.
I'm not a betting man, but the odds are not good.
Can we talk about this later?
Someday, this'll all be a bad memory.
I'm not even gonna tell you to shut up.
When this is all over, I'm gonna mate.
Wouldn't be the first time.
Whoa…. Déjà vu!
Doesn't bear thinking about it.
They're never going to make a Stalker out of me.
There's always a first time. Or, Ill put it on your Tomb Stone.
God I'm hungry.
Lets concentrate on the task at hand.
If I ever get my hands on Dr. Breen….
That's more information than I require.
I think I ate something bad.
Yeah, you and me both.
Sometimes, I dream about cheese.
You should nip that kind of talk in the bud.
Finally, change is in the air!
Keep your mind on your work.
You feel it? I feel it!
I wish I had a dime every time somebody said that.
I can't remember I the last time I had, well, anything.
I won't hold it against you.
When this is all over, I'm…. aw, who am I kidding?
Try not to let it bother you.
I'm pretty sure this isn't part of the plan.
Same here.
Doesn't anyone care what I think?
Hey, that's enough out of you.
I just knew it was gonna be one of those days.
I wouldn't say that too loud.
Looks to me like things are getting worse, not better.
What am I supposed to do about it?
This is bullshit!
Try not to dwell on it.
I don't dream anymore.
That's you all over.
I'm glad there are no kids around to see this.
I'm not even gonna tell you to shut up.
If I could live my life over again.
Figures.
I'm not even gonna tell you what that reminds me of.
Your mind is in the gutter.
I can't get this tune out of my head….Woo, woo woo, woo woo woo-woo woo.
Wanna bet?
I can't believe this day has finally come!
No argument there.
I could eat a horse, hooves and all!
Have you ever had an original thought?
You smell that? It's freedom.
Why are you telling me?