In-game just press 'T' and then enter the following codes:
Code - Effect
mpimyourfather - God Mode
mpwegotdeathstar - Infinite Ammo
mpkingoftehmonstars - All Weapons & Full Ammo
mpmaphole - Complete Mission
mpdrdentz - Full Health
mpwonderbra - Full Armor
mpyoulooklikeyouneedamonkey - Add All Armor Options
mpgoattech - All Weapon Upgrades
mpmiked - Exit Game
mpbuild - Show Game Build
mpasscam - 3rd Person View
mprosebud - Spawn Snowtruck
mppos - Toggle Position Display