Invincibility:
Pause the game and press Square, Circle, Left, Right, Circle, Square.

All weapons:
Pause the game and press Left, Circle, Right, Square, Right, Left. [send by:alex]

Level skip:
Pause the game and press Up, Down, Left, Right, Right, Left, Circle.

Level select:
Save the game and return to the main menu. Select the "Resume Game" option, then highlight the previously saved game and press Up, Down(2), Up, Left, Up, Right, Down.

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.

Fatboy Mutator:
At the title screen, press Circle(3), Up, Down(2), Up, Circle(3). Select multi-player mode and choose the Fatboy Mutator. Get killed to have the new Mutator get thinner. Get a kill to have the Mutator get bigger.

Stealth Mutator:
At the title screen, press Square(2), Circle(2), Square(2), Circle(2). Select multi-player mode and choose the Stealth Mutator.

Firewire multi-player mode:
Note: This trick requires a four port Firewire PC hub. Use Firewire cables to connect the iLink port of one to three more PlayStation 2 consoles to the hub. Pause the game in two player multi-player mode and press Left, Circle, Left, Right, Square, Right. Have all players press Start at the waiting screen.

USB multi-player mode:
Note: This trick requires a USB keyboard and USB mouse. Change the controller configuration to set the keyboard to player three and the mouse to player four. The game can now by played with four players.