If you've got the unpatched US version, try these cheats during gameplay:
biggrenade:homing sniper grenade
power-up
decoykurt:decoy power-up
makemefull:100% health
masterblaster:super chaingun power-up
twistandshout:twister power-up
486willbeslow:lets game run on 486s
Note: Each code can be used only once per level.