Matrix Mode (slo mo):
Begin the game as Max, then hold R2 and press Up 4 times.

Lounging Kurt:
At the main menu hold L2 + R2 and press , , , .

Kurt In His Boxers:
At the main menu hold L1 and R1 and press , , , . Remove menu from pause screen. Pause the game and hit L1 and R1.

A Hydra MDK Style:
Pause the game, then hold L2 + R2 and press Up, Down, Right, Left, , , , , + Start to play as a character that is part Kurt, Max, and Doctor Hawkins.

Panning Camera:
Pause game play, then hold L2 + R2 and press , , , .

Crazy Camera Views:
Pause the game, then hold L2 + R2 and press , , , .

Dr Flagulence...er Doctor Hawkins:
Begin the game as Doctor Hawkins, then press L2 + R2 + Left + .

Invincibility for All Characters:
While at the pause screen enter while holding R2 and L2: Up, Up, Down, Down, Left, Left, Right, Right, SELECT (hold until pause screen disapears). *This code goes away after the level that you entered it on is over, so you must re-enter it for every level.

Thanks to Revolution readers Grimlok, Corey J, MADDsk8er1288 and David Pelier!