While playing in single player (Campaign) mode, drop the multiplayer chat window to enter cheats. The controls in game options will have the default keybind for this function.

turnpunchkick - 2D characters
1-900 - Large torsos
meganoggin - Big heads
bignoggin - Big heads (alt)
5fingerdiscount - Restock ammo and items
explore - Objectives on/off
nobrainer - No target mode (AI on/off)
stumpy - Midget characters
avatargod - God mode (selected character)
teamgod - God mode (team)
silentbutdeadly - Farting sounds
fastactionresponseteam - Farting sounds (alt)
clodhopper - Large appendages
teamshadow - Invisibility (team)
theshadowknows - Invisibility (player)
debugkeys - Allows for debug commands
Debug Commands
F2 - Screen capture
F6 - Enemy view (cycle)
F7 - Kill self
F8 - Kill self
F9 - Normal view (player)
F10 - Exit debug mode (use code to return)
F12 - Skip level
[ or ] - Gamma adjust
A - AI toggle
V - Third person view
B - Third person view
, or . or / - Z axis adjust