Press 'Enter' during game play then type in the following cheats:

avatargod               God mode
teamgod                 Team God Mode
explore                 Turn off victory mode
5fingerdiscount         Restores your ammo
nobrainer               Turns of AI
turnpunchkick           Players 2d
meganoggin              Bighead mode
bignoggin               All players have a big head
clodhopper              Big feet and hands 
stumpy                  Stumpy mode
1-900                   Heavy Breathing

debugkeys               Debug Keys On

     When entering debugkeys, press F10 to enter debug mode. These are
the keys that work in debug mode.

F2                      screenshot
F6                      change view
F7 or F8                instant death
F9                      return to normal view
F10                     toggle debug mode
F12                     skip level
[ or }                  adjust lighting
1 and 3 on keypads      change screen resolution
A                       toggle AI
V or B                  rotates view