Kariya               Max out Ammo
corbet or stilllife  Be invisible
drury                Get full ammo
roy                  invincibility
domi                 artiltery strik
thetrooper           God Mode            
diewithyourbootson   Unlimited Ammo      
sunandsteel          Reload Ammo  
revelation           Artillery Rounds 
Bozo                 Bozo Mode  
roy                  Invincibility  
fly_up               Fly up   

Commands:
/s:                  Save Scores
/d:                  Reload From Directory, Not PIF file.
/p filename:         Play Demo
/sys:                System Info.
/?:                  Display Command Options
/r:                  Record Demo In 1-Player Mode.  

Hints

Magical Reload:
If you save your Mission, when you reload the Mission, your Ammo is back up tp Full...

Loading & Saving:
Type in savegame or loadgame to do one or the other.

Mobile Machinegun and/or Grenade Launcher:
Throw away your primary weapon (by pressing 0) and then go to some machinegun or grenade launcher. Then go away and the machinegun (or grenade launcher) will stay in your hand so you can shoot from it while moving. If you change your weapon or pick up another primary weapon, the machinegun (or grenade launcher) will be lost and you have to repeat this action in order to get the machinegun (or grenade launcher) back.

Thanks to Revolution readers sadakosan, Lasse Treland, Tristan de Jong, Spyder and Kenneth Barrett!