Go to extras and enter down, Right, Down, Right, Up, Down, up.
Restore Health and gain 100% Armor:
During gameplay press R, R, R, L, R, R, L, R.
Unlock ALL Eye Powers:
During gameplay press L, L, R, R, R, R, L, L.
Unlock ALL Stages in Multi-Player:
At the Main title enter L, R, X, Y, X, Y, L, R, X, X, Y, Y, Z.
Unlock ALL Artworks:
At the Extras menu press Down, Right, Down, Right, Down, Up, Down, Down, Right, Down, Right, Down, Up, Down, Left.
Unlock One Life Mode
Beat the game on hard difficulty to unlock one life mode.
Unlock All Skins in MP Mode
At the Extras menu, enter the following code:
Down, Left, Up, Left, Right, Down, Left, Up.
Unlock Paintball Mode
At the Extras menu enter the following code:
Right, Left, Right, Left, Down, Down, Up, Up.
Unlock All Single Player and Multi-player Levels
At the Extras menu, enter the following code:
Down, Right, Down, Right, Up, Down, Up, Left.