To activate cheats press ' in game and type them in:
Code - Result
teamgod - Team God Mode
avatargod - Player God Mode
stumpy - Stumpy Mode
clodhopper - Enlarges Player's Feet and Hands
meganoggin - Mega Head Mode
bignoggin - Big Head Mode
5fingerdiscount - Refill Ammo
nobrainer - Turns AI Off
debugkeys - Debug Keys Enabled
silenbutdeadly - Flatulent walking
turnpunchkick - Changes Players from 3D to 2D
1-900 - Heavy Breathing
explore - Victory Conditions On/Off

Debug Mode
While playing, type " then type 'debugkeys'. To activate it hit ENTER, press F10, then press any of the following keys:

[F12] - Level skip
[F7] or [F8] - Suicide
[F6] - Change view
[Comma], [Period], or [/] - Adjust elevation