To enable the following codes, you must first beat the two levels listed for each code on Hardcore difficulty. Then enter them at the Start menu:

Fully Equipped:
R1, R2, L1, L2, Down, Up, Left, Up. Beat Drunk Driving and Graveyard Shift levels first.

Invisible:
Square, Square, Square, Down, Square, Down, Circle, Up. Beat Trained To Kill and Border Patrol levels first.

Regeneration:
R2, Right, Circle, R2, L2, Down, Circle, Left. Beat Fueled By Hate and Grounds For Assault levels first.

Super Punch:
L1, Triangle, Triangle, Triangle, Circle, Circle, Circle, R1. Beat Mouth Of Madness and Doing Time levels first.

Monkey Skin:
Square, Square, R2, Down, Triangle, Square, Circle, Down. Beat Press Coverage and Wrong Side Of The Tracks levels first.

Rabbit Skin:
Left, R1, R1, Triangle, R1, R1, Square, L1. Beat Kill The Rabbit and Divided They Fall levels first.

Pig Skin:
Up, Down, Left, Left, R1, R2, L1, L1. Beat Key Personell and Deliverance levels first.

Thanks to Revolution readers Raging Flippy, AphexTwin, and Mr. Murder!