These cheat codes do NOT work in multiplayer mode. Entering any cheat code will reset your score to 0 and disable the score counter. 

ALIFALAFEL   all accessoires (without weapons) 
ALMIGHTY     invulnerability on / off 
DELSHIFTB    destroys the reactor and set you (with all weapons of the level) to the exit 
DUDDABOO     all weapons bounce (like the Phoenix cannon) 
FREESPACE    warp to other levels (you can enter the level number) 
GODZILLA     bumping into robots will kill them 
GOWINGNUT    the Guide-Bot gets angry and shoot to every robot (be careful when it shoots Mega Missiles!) Enter again to calm him down again. 
HELPVISHNU   another Guide-Bot 
HONESTBOB    all weapons (due to a bug in V1.0 this cheat will give you only the shareware weapons in that version 
LPNLIZARD    homing weapons 
ORALGROOV    all keys 
ROCKRGRL     Full Map 
SPANIARD     all robots are toasted (killed) (the boss robot will be killed when entering one more time and the next time the guide-bot will be toasted (sniff)) 
BLUEORB      only V1.1+: shield boost (same as collecting a shield powerup) 
IMAGESPACE   only V1.1+: Robots stop shooting 
SILKWING     only V1.1+: the robots kill each other 
WILDFIRE     only V1.2+: very fast shooting 

The following are non-cheating commands. They are also available in multiplayer mode, since they are no real cheats, meaning they give no advantage to the player that uses them.

BITTERSWEET    Gives you a funny view as if you are looking through a fish-eye-lense... Entering it again will undo it 
GABBAGABBAHEY  The cheat from Descent 1 is still available, although its result is not the same: your shield and energy will be set to 1 (!) and in multiplayer game everyone will also be notified of your bad try :-(! Did you really think, Parallax didn't change the cheat codes as id Software did it for Doom/Doom 2? 
PIGFARMER      Replaces the background texture you see when playing with smaller windows (press "-") with a photo of a programer of Parallax Software. Entering it again will undo it. 

Non-cheating commands will a) not activate the cheat mode, meaning you still are able to score and b) they are also available in multiplayer mode. 

Note: Do better not try to use the old GABBAGABBAYHEY cheat code or one from Descent 2 Demo, since most of them actually do work, but not showing the result you wished: they handicap you to 1 shield and 1 energy.