All cheats enabled:
Enter BOBBYBIRD as a code.

Level select:
Enter THEEARTHMOVED as a code.

Invincibility:
Enter ASSMAN as a code.

All weapons:
Enter FATKNACKER as a code.

Unlimited ammunition:
Enter FATTERKNACKER as a code.

All characters in multi-player mode:
Enter OMGTKKYB as a code.

Officer Barbrady:
Enter ELVISLIVES as a code.

Mr. Mackey:
Enter CHEATINGISBAD as a code.

Phillip:
Enter PHAERT as a code.

Terrance:
Enter RAFT as a code.

Mr. Garrison:
Enter DOROTHYSFRIEND as a code.

Chef:
Enter LOVEMACHINE as a code.

Wendy:
Enter CHECKATACO as a code.

Pip:
Enter FISHNCHIPS as a code.

Ike:
Enter KICKME as a code.

Mrs. Cartman:
Enter ALLWOMAN as a code.

Mephisto:
Enter GOODSCIENCE as a code.

Jimbo:
Enter STARINGFROG as a code.

Ned:
Enter HAWKING as a code.

Big Gay Al:
Enter OUTRAGE as a code.

Alien:
Enter MAJESTIC as a code.

Starvin Marvin unlocked:
Enter SLAPUPMEAL as a code.

Skinny mode:
Enter VEGGIEHEAVEN as a code.

Big head mode:
Enter MEGANOGGIN as a code.

Pen and ink mode:
Enter PLANEARIUM as a code.

View credits:
Enter SCREWYOUGUYS as a code.

Hint: Defeating Ultra Mega Mega Man:
Whenever Ultra Mega Mega Man run backs to get more health, go over to the platform where he charges up. Hit the button on the right when it starts blinking to prevent him from recharging.

When fighting Ultra Mega Mega Man, shoot him three times with the Super Sniper Chicken (which is provided for you in his "lair"). When he starts to spaz out and lower his head, wait until you see him walking towards the power station in the corner of the room. Get right behind him, and after about three to four eggs to his power pack, Ultra Mega Mega Man will be no more. This way, you do not have to keep shooting the large octagon-shaped button to stop his recharge process. Even if you do this, you will still have to shoot him in the back anyway, and he will still try to recharge.

More speed:
Hold C-Up and press C-Left or C-Right to run at an angle at higher speed.