Press ~ during game play to display the console window, then activate the following cheats by entering the corresponding codes:

CHEAT FUNCTION - CHEAT CODE
Spawn indicated item and number - give <1-200>
All weapons - gimme
No clipping mode - noclip
God mode - god
Flight mode - fly
Disable flight and no clipping - walk
All weapons - allweapons
No clipping mode - ghost
Full ammunition - allammo
Toggle invisibility - invisible <0 or 1>
Kill everything - killall
Get everything - loaded
Toggle enemies cannot see - blindenemies <0 or 1>
Toggle AI cannot hear - deafai <0 or 1>
Toggle AI cannot see - blindai <0 or 1>
Toggle enemies cannot hear - deafenemies <0 or 1>
Unknown - unloaded
Unknown - loadspmap
Unknown - supersquad
Unknown - oldmovie
Unknown - lightperiod
Unknown - lightcone
Unknown - getfirstmission
Unknown - pawnanimextra
Unknown - soundocclusion
Unknown - smiteevil
Unknown - avatar
Unknown - summon