Pause the game and enter these codes:

boomstix	- All weapons and ammo
donnytrump	- Gold and stat tokens
elvis		- No clipping mode
gps		- Display coordinates
gripper		- Stealthboots
jimmy		- All keys
lego		- Get a piece of Sigil
omnipotent	- Invincibilty
pumpup<X>	- Power up, where X is first letter of powerup. 
rift<##>	- Level Select, where ## is level number.

Thanks to Revolution readers Monica Davis and Jhony!