Master Cheat:
Enter BOBBYBIRD at the cheat menu to activate all cheats. 

All characters in multi-player :
Enter OMGTKKYB at the cheat menu. 

All Weapons:
Enter FATKNACKER at the cheat menu. 

Unlimited Ammo: 
Enter FATTERKNACKER at the cheat menu.

Bonus Characters:
Enter each code in the cheat menu: 
cheatingisbad = Mr. Mackey
elvislives = Bar Brady 
outrage = Big Gay Al 
hawking = Ned 
slapupmeal = Starvin Marvin
phaert = Phillip 
raft = Terrance 
dorothysfriend = Mr. Garrison 
lovemachine = Chef 
checkataco = Wendy 
fishnchips = Pip 
kickme = Ike 
allwoman = Mrs. Cartman 
goodscience = Mephisto 
staringfrog = Jimbo 
majestic = Alien 

Invincibility:
Enter the password ASSMAN at the cheat menu. 

Level Select:
Enter the password THEEARTHMOVED at the cheat menu. 

Skinny Mode:
Enter the password VEGGIEHEAVEN at the cheat menu. 

Big Head Mode:
Enter the password MEGGANOGGIN at the cheat menu. 

Pen and Ink Mode:
Enter the password PLANEARIUM at the cheat menu. 

View Credits:
Enter SCREWYOUGUYS at the cheat menu.