While playing the game, press Left(4), Black, Right(4), White to unlock all levels.
While playing the game, press Up(2), Down(2), Left, Right, Left, Right, B, A to spawn a medpak that will restore 25 points of health. -From: email@example.com and firstname.lastname@example.org
While playing the game, press Y, B, A, X, White, Y, X, A, B, White to spawn an ammunition cartridge for your current weapon.
When you need to run a long way and you have some objects with you (for example, a non-explosive barrel), if you have the Gravity Gun, 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 it with one shot.
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.
Pick up an item 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.
When you get the jeep, use turbo towards a wall, then press Y to get out before you hit the wall. If done correctly, when the jeep flips you will also flip. Note: You may also get hurt.
When you are fighting alongside Alyx later in the game, 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 will appear 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.
After Dog attacks the dropship and it drives off, go to the bent down fence and jump over it. Walk down a few steps and on your right will be 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. 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.