Level select
While playing the game, press Left(4), L1, Right(4), R1 to unlock all levels.
Restore health
While playing the game, press Up(2), Down(2), Left, Right, Left, Right, Circle, X to spawn a medpak that will restore 25 points of health.
Refill ammunition for current weapon
While playing the game, press Triangle, Circle, X, Square, R1, Triangle, Square, X, Circle, R1 to spawn an ammunition cartridge for your current weapon.
No clipping mode
While playing the game, press Up(3), L1, Up(3), R1, Up(2).
Enemies do not target you
While playing the game, press Down(3), L1, Down(3), R1, Down(2).
AA
While playing the game, press Triangle, Up, Triangle, Right, Triangle, Down, Triangle, Left, Triangle, Up.
Alternate title screen
Successfully complete the game to alternate the title screen.
Getting past turrets and other dangerous enemies
When you need to run a long way and 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 is only one enemy when doing this. Also, make sure the object is indestructible, and most importantly, look at the enemy and see if you can kill it with one shot.
