Press the shift and the @ key or the ~ key on some systems to make the Game Console Window Appear.
Type in SISSY to enable the cheat modes then enter any of the following below for goodies...

(Code - Effect)

Osama - All Non-Players become Fanatics
Whatchutalkinbout - All Non-Players become Gary Coleman
SwimWithFishes - All Radar Items
BlockMyAss - Body Armor
RockinCats - Change Guns to have Cats on them
IAmTheLaw - Cop Uniform
walk - Disables flight and ghost modes
splodincats - Disables ricocheting cats
slomo 1 - disables Slow-Motion
Whatchutalkingabout - Everyone is Coleman
LikeABirdy - Fly Mode
Healthful - Full Health, 4 Medkits
PacNHeat - Get Every Destructive Weapon
IFeelFree - Ghost Mode
SmackDatAss - Gimp Suit
Alamode - God Mode
PayLoad - Lots of Ammo in All Weapons
JewsForJesus - Lots of Cash
IAmTheOne - Lots of Catnip
LotsA♥♥♥♥♥ - Lots of Cats
BoyAndHisDog - Lots of Dog Treats
lotsadogs - Lots of Dogs
PiggyTreats - Lots of Doughnuts
Jones - Lots of Health Pipes
iamsolame - Maximum ammo, invincibility, and all weapons
Ghost - No Clipping (Walk Through Walls)
headshots - One shot kills
DokkinCats - Removes Cat Repeating Guns
Setday - Reset Day to Monday
FireInYourHole - Rocket Cameras
nowwedance - Scissors machin gun
slomo - Slow Motion
playersonly - Stop Every Non-Triggered Action
boppincats - When gun is shooting cats, cats will ricochet