Cheat mode:
Enable cheats by first entering the password NRYRDDS. Now enter the following passwords:

ALL      God Mode 
AMMO     Unlimited Ammunition 
HEALTH   Unlimited Health 
KEY      All Keys 
MAPS     All Maps 
PWORD    Disables cheat mode and enable "special passwords" 
SKIP     Level Skip 
SFX      Sound Effects Test 

God mode:
(Special password) - Enter the password S_X_N to access everything.

Thanks to Revolution readers Anthony, Martin Brewer and Patrick!