While playing, press ` (tilde key) to enter the console, then type: 

Code                     Result 
Invuln                   God Mode 
GimmieGimmieGimmie       All Weapons 
NoClip                   No Clipping Mode 
NoTarget                 Monster Targeting On/Off 
Spectator                Fly Mode 
ChaseCam                 Chase Camera (5 Different Angles) 
CyclePlayerClass         Cycle Through 4 Player Classes 
NextArmor                Increase Armor and Health 
PrevArmor                Decrease Armor 
NextMonster              Go to Next Monster 
PrevMonster              Go to Previous Monster 
RestartLevel             Restart Current Level
listcommands or set      List console commands