Skip to Next Level
Pause the game and press Up, Down, Left, Right, Right, Left, Circle.
Level Select
Save your game and go back to the main menu. Once you are at the resume game screen, highlight your saved game and press Up, Down, Down, Up, Left, Up, Right, Down.
Unlock Fatboy
Go to the title screen and press Circle, Circle, Circle, Up, Down, Down, Up, Circle, Circle, Circle. Now go to the Multiplayer Mode and look for a new Mutator called Fatboy. Turn this on and your character will become fatter for every kill you make and skinnier for every time you get killed.
Infinite health
Pause the game and press Square, Circle, Left, Right, Circle, Square.
Maximum ammunition
Pause the game and press Left, Right, Circle(3), Right, Left.
Stealth Mutator
At the title screen, press Square(2), Circle(2), Square(2), Circle(2). Select multi-player mode and choose the Stealth Mutator.
Four Players without MultiTap
All you need is a "USB" keyboard and mouse to add an extra player. This will also allow you to play four player via two keyboards and two controllers. Set the controller configuration such that the keyboards and mouse's are in the third and fourth player positions.
Change Camera View
Hold Circle + X + Start while playing the game. The game will pause. Resume the game with the view fixed.
To return to First Person View, hold Triangle + Square + Start while playing the game. The game will pause. Resume the game with the new view.
Gatling Gun From Wolfenstein
To get the Gatling Gun from Wolfenstein, simply press and hold SELECT and then press Up, Up, Down, Down, Left, Right, Left, Right, Square, Triangle, Square, Triangle, and START at the Title screen.
God Mode
During the game you can press the START button and enter:
LEFT, RIGHT, CIRCLE, CIRCLE, CIRCLE, RIGHT, LEFT.
Once you hit the last button you'll be taken back to the game, but this time you're invincible.
Big Head Mutator
At the Main Menu press: Left, Right, Left, Right, Left, Right, Circle, Circle, Circle.
Now, enter the multiplayer option. Select how many players then at the level select screen you can highlight the MUTATORS option and hit X. Highlight the big head option and press RIGHT. Now press the X button and enter the game.
All Characters
Press Start to pause the game, then press Left, Left, Circle, Circle, Right, Left, X.