Half-Life 2

Spawn medpak:
Press Up(2), Down(2), Left, Right, Left, Right, B, A during game play to spawn a medpak that will restore 25 points of health.
Anthony Guthrie.

Spawn ammunition cartridge for current weapon:
Press Y, B, A, X, White, Y, X, A, B, White during game play.
theghost and Chunks48.

Chapter select:
Press Left(4), Black, Right(4), White during game play. Return to the main menu and start a new game to have all chapters unlocked.
birevdtpia rtap, .

Hint: Follow Freeman: Item room:
You will hear the metal device where you get two people to follow you. After it is done, you will see a room to go through. You can find items there, but be careful; sometimes you may die. Save the game before entering it.
Nathan Wallace.

Hint: We Don't Go To Ravenholm: Hidden items:
After climbing over the white cars and going through the window, you will go in to a room that has two windows and a zombie in it. When you climb out the window, you will see the old man with a shotgun in a window. Proceed to your left on to a concrete ledge. You will see a flammable barrel behind a grate. Shoot at it and the grate will blow out. Hidden back behind the grate is health and ammunition.
Eric Coz.

Hint: Getting past turrets and other dangerous enemies:
When you need to run a long way in front of you and you have some objects with you (for example, a non-explosive barrel), if you have the gravity gun, pick it up. Use it as cover and run (while facing the enemy) to the other side. Then if possible, shoot the barrel at the enemy. Note: Make sure there are no more that one enemy while doing this. Also make sure that the object is indestructible, and most importantly, look at the enemy and see if you can kill him or it with one shot.
Joe Palka.

Hint: Punting objects:
Pick up an item in the game without using the Gravity Gun. Drop it until it leaves the bottom part of the screen. Then, pick it back up and throw it as fast as possible. If done correctly, it will appear as if you are kicking the object. This is especially fun to do with the toy doll at the beginning of the game. Note: This works best with lighter objects.

Hint: Flip jeep:
When you get the jeep, use turbo towards a wall. Press Y to get out before you hit the wall. If done correctly, when the jeep flips you will also flip. Note: You may get hurt as well.
Nathan Wallace.

Hint: Re-activating turrets:
To re-activate a turret, crouch and face the way the legs are sticking out from the turret. Pick it up with your gravity gun or use key, and face up while you have the turret and it will re-activate. When there are no enemies around, carefully press Y (or L if you have gravity gun) until the turret stands up straight and is activated. Note: This is important if you are using turrets to kill the CPs coming from the blue openings. Never re-activate enemy turrets that you disable.
Joe Palka.

Hint: Saving ammunition:
In the "We Don't Go To Ravenholm" level, use the gravity gun to pick up the jagged metal discs. You can fire these at enemies for an instant kill. Also if the enemies are in a line it will kill all of them in one shot. This helps mostly against the zombies and will allow you to save ammunition for later levels.
craig green.

Hint: G-Man sighting:
At the beginning of the game a man (G-Man) tells you to wake up. Well on the "Water Hazard" chapter, after beating the helicopter you will open the gate. There is a building with supplies, grenades, ammunition, and health. After going there, go down the grassy area to the right of it. In the distance you can see a concert ledge where you must go to next. Zoom in and try to spot the man standing there in a gray suit holding a briefcase. He will only appear briefly before walking behind a wall.
Dylan Carter.

Hint: Movie 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).
Dave.

Hedy Lamarr was a famous Austrian actor in the U.S. from the 1930's through the 1950's. Mel Brooks later claimed that Hedy Lamarr tried to sue the producers for the "Hedly Lamarr" jokes. Dr. Kliener's Headcrab was actually names after the "one and only Hedy Lamarr".
Lawrence Huttenhoff.

Hint: 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.
Matthew020192.

Note: Whenever the logo appears, there will be health, ammunition, or other pickups available somewhere around that area. The items may sometimes be hidden.
Mark Harris.

Glitch: Clone Alyx:
Later in the game when you are fighting alongside Alyx, at one point you will encounter a door where she tells you to go on without her, while she heads up to the communications room to unlock doors for you. Push her into the corner left of the door, then run in and watch it close behind you. As the next area is loading, she appears next to you. This clone does not talk, and is not always there. However, later when both of you fight a group of soldiers, there are two of them.
Mikemc.

Glitch: Anticitizen One: Get on top of stairwell:
After Dog attacks the dropship and and it drives off, go to the bent down fence and jump over it. Walk down a few steps and on your right is an open door. Go in it and to your left you will see a stairwell blocked by a couch and debris that cannot be moved. Crouch jump over the couch and go up the stairwell. You might want to bring a box with you so that you can put it on the ground in front of you then jump on top of it and make longer jumps.
mellowyellow685 Singleton.

Glitch: Highway 17: Edge of world:
Immediately after the Combine outpost with the crane, you will jump a gap in the buggy and get attacked by a gunship. There is a crate full of rockets in the van in the middle of the road you can use to shoot it down. Clear the road with the gravity gun and drive the buggy through. After passing two more telephone poles, there is an opening on the left with another broken-down van and a rock structure on top of the hill that vaguely resembles Stonehenge. This van has three crates with medkits inside if you need them. Drive the buggy past the van and straight at the rock formation. It should climb the wall and allow you to walk to (or off) the edge of the world. Note: You will die if you walk off. Also, there are some invisible walls as soon as you get up there, but you can travel a number of miles along the ridge (but no farther along).
Darien Fawkes.

Glitch: Nove Prospect: Death by looking at wall:
Get to the end of Nove Prospect where the wall comes in and you need to get to higher grounds by the fire. At the location where a Head Crab comes at you, turn to face the wall and look for a opening go through. You will die.
Nathan Wallace.

Debug mode:
Press Start, Up(6), Down, Start, Up(3), Down during game play.
This code has been reported as inaccurate. Please submit any corrections or verifications.

Unlimited health:
Press Up, Down, X, Y, B, Down(4), Up(4) during game play.
This code has been reported as inaccurate. Please submit any corrections or verifications.

Inorganic Gravity Gun:
Equip the SMG, then press Down, Up(2), Down, L, R, X, B, A during game play. Note: This code cannot be enabled with the "Organic Gravity Gun" code.
This code has been reported as inaccurate. Please submit any corrections or verifications.

Organic Gravity Gun:
Equip the Inorganic Gravity Gun, then press L, Black, R, White, Up, Down, Y, X, B, A during game play. Note: This code cannot be enabled with the "Inorganic Gravity Gun" code.
This code has been reported as inaccurate. Please submit any corrections or verifications.