Unlimited ammunition:
Press X, Triangle(3), X, Circle, X, Square at the code entry screen.

Invincibility:
Press Square(2), X, Triangle, X, Circle, X, Circle at the code entry screen.

All future weapons:
Press X(3), Triangle, Circle(2), Square, X at the code entry screen.

All past weapons:
Press Circle(2), Triangle, Square, X, Triangle(2), Square at the code entry screen.

Quadruple damage:
Press Triangle(4), Square, X, Circle, Square at the code entry screen.

Automatically complete missions:
Press Square(2), Triangle, Circle(2), X, Square, Triangle at the code entry screen. 

Fight TX Endoskeleton in all battles:
Press Triangle, Square, Triangle, X, Triangle(4) at the code entry screen.

50% more health in combat with TX:
Press Triangle(2), Square, X, Circle, Triangle, Square, Circle at the 
code entry screen.

50% more health for TX in combat:
Press Square, Triangle, Square, Triangle, Circle, X, Circle, X at the 
code entry screen.

Centipede mini-game:
Successfully complete the game to unlock Centipede at the "Atari Games" menu. 
Note: If it is not unlocked as soon as you complete the game, restart your 
PlayStation2, reload your game, and recheck the "Atari Games" menu.

Missile Command mini-game:
In the vet's office on level 13, walk into the small office on the far right side. 
Approach the computer and press X to play Missile Command. Then, successfully 
complete the level to unlock the game at the "Atari Games" menu.