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.