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
theshadowknows         Invisible mode
teamshadow             Team invisible mode      
debugkeys              Enable Debug mode...
silentbutdeadly        Fart mode kills all 
fastactionresponseteam Fart but no kills 
monocle                Monocle mode 
1-900                  Heavy breathing 
stumpy                 Stumpy Mode 
normalcolors           Map draw AI normal colors 
panickedcolors         Map draw AI panicked colors 
roestatecolors         Map draw AI ROE colors 
roespeedcolors         Map draw AI ROE speed colors 
combatstatecolors      Map draw AI combat state colors 
behaviorcolors         Map draw AI behavior colors 
psychstatecolors       Map draw AI psychological state colors 
threatawarenesscolors  Map draw AI threat awareness colors 
awarenessstatecolors   Map draw AI aware colors 
alertnessstatecolors   Map draw AI alertness colors 
aiplancolors           Map draw AI plan colors 
levelmaps              Map draw level maps 
testplan               Map draw test plan 
drawplan               Map draw plan 
pathpointlinks         Map draw path point links 
coverpoints            Map draw cover points 
pathpoints             Map draw path points 
objectwalls            Map draw object walls 
obstaclewalls          Map draw obstacle walls 


Press F10 to toggle debug mode. Now press the following:

A                       Artifial Intelligence On/Off 
M                       Freezes you
V                       Rotates View
B                       Normal view
1 and 3 on keypad       Change resolution
- and + on keypad       Zoom
F6                      Look through the eyes of other people in the mission
F7                      Automatically die
F9                      Return to the player you were using to complete the mission
F12                     Level Skip 
[ and ]                 Brighten and darken the screen
< and >                 Make the player float up or down
/                       Brings you down from floating