Pressing " D " and then " N " activates cheat mode. Then enter any of
the following codes: 

CORNHOLIO
Turns God mode on or off and gives you unlimited jetpack fuel 
KROZ
Turns God mode on or off and gives you unlimited jetpack fuel 
COSMO
Gives you unlimited jetpack fuel 
STUFF
Gives you all the weapons, keys and full ammo 
ITEMS
Gives you all of the items and keycards 
SCOTTY # (episode) # (level)
Warp to a level 
CASHMAN
Makes you spew money 
VIEW
Gives you a third-person view of Duke 
RATE
Displays current frame rate 
SKILL#
Changes the skill level 
HYPER
Get high on steroids 

Type "D","N", then WEAPONS. This is similar to STUFF, but without the
items (Jetpack, Medkit, Steroids, etc,.)