Pause the game and enter the following codes. NOTE: These will not work in Xbox Live games.

16,000 credits:
Up, Up, Down, Down, Left, Right.

Low gravity:
Up, Right, Right, Up, Up, Left.

Untargetable by enemies:
Left, Right, Right, Left, Up, Down.

Identify friends or enemies:
Right, Right, Right, Up, Down, Left.

Big head mode :
Up, Up, Left, Right, Up, Down.

New costume:
Left, Right, Down, Up, Left, Right.

Player clipping:
Up, Down, Left, Right, Down, Up.

Toggle auto-aim:
Down, Down, Down. Down, Up, Up.

Thanks to Revolution readers Big Toe and mountsander!