Press Esc during gameplay and choose Options. Go to the bottom left-hand corner of the screen and click until you hear a sound. Now type:

BEEFCAKE         Invincibility
SWEET            All weapons and Infinite ammo
FRAMERATE        Display the framerate
EGOTRIP          Big head mode

Thanks to Revolution readers ed@ie, Naoj, Phil, and Edward Mcgowan 3a2!