Enter the following during play. Enter again to disable:

Invisibility:
Triangle, X, Square, Circle, Circle, Left

Slow motion:
Triangle, X, Square, Circle, Circle, Right

Fast motion:
Triangle, X, Square, Circle, Circle, Down

Max charisma:
Triangle, X, Square, Circle, X, Down

Infinite ammo:
Triangle, X, Square, Circle, X, Right

Sub-machine gun and various items:
Triangle, X, Square, Circle, Triangle, Up

Sniper rifle and various items:
Triangle, X, Square, Circle, Triangle, Right

Rocket launcher and various items:
Triangle, X, Square, Circle, Triangle, Left

Heavy machine gun and various items:
Triangle, X, Square, Circle, Triangle, Down

Shotgun and various items:
Triangle, X, Square, Circle, Circle, Up

Nail gun:
Triangle, X, Square, Circle, X, Left

Thanks to Revolution readers Barnaby Shapiro and Wallace!