Different Credits:
Go to the "Press Start" screen and press X, Triangle, Square, Circle
All Cheats:
Go to the "Press Start" screen and press Square, Circle, L2, R2
Get 2D Mode:
Go to the "Press Start" screen and press Down, Up, Circle, Square
Big Head Mode:
Go to the "Press Start" screen and press Square, Circle, Up, Down
More Boxer Textures:
Go to the "Press Start" screen and press L1, R1, X, X, Triangle, X
Tiny Head Mode:
Go to the "Press Start" screen and press Square, Circle, Down, Up
Mutant Mode:
Go to the "Press Start" screen and press Square, Left, Up, Triangle