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



Hints

Dont Be Affected By Flashbangs:
When a flash is thrown, quickly turn on night vision and the flashbang will not effect
you