Press "T" while playing then type in one of the following...
MPGOD - God mode
MPKFA - Lotsa weapons
MPAMMO - Full ammo
MPCLIP - Spectator mode (no clipping)
MPHEALTHY - Fills up your health
MPWHEREAMI - Displays your coordinates
MPHIDEME - Hides your coordinates
MPBEEFCAKE - Increased Power (more gore)
MPKILLEMALL - Kill's all monsters on level
MPSPEEDUP - Increases your speed (1..5)
MPSTRONGER - Increases your strength (1..5)
MPCALEB - Changes your character model to Caleb
MPOPHELIA - Changes your character model to Ophelia
MPISHMAEL - Changes your character model to Ishmael
MPGABBY - Changes your character model to Gabriella
MPBERETTA - Gives you the Beretta, type twice for dual weapon
MPSUBMACHINEGUN - Gives you the Submachinegun,twice for dual weapon
MPFLAREGUN - Gives you the Flaregun,twice for dual weapon
MPSHOTGUN - Gives you the Sawed-off Shotgun, twice for dual weapon
MPSNIPERRIFLE - Gives you the Sniper Rifle
MPHOWITZER - Gives you the Howitzer
MPNAPALMCANNON - Gives you the Napalm Cannon
MPSINGULARITY - Gives you the Singularity Generator
MPASSAULTRIFLE - Gives you the Assault Rifle
MPBUGBUSTER - Gives you the Bug buster
MPMINIGUN - Gives you the Minigun
MPLASERRIFLE - Gives you the Cobalco Laser Rifle
MPTESLACANNON - Gives you the Tesla Cannon
MPVOODOO - Gives you the Voodoo Doll
MPTHEORB - Gives you the The Orb
MPLIFELEECH - Gives you the Life Leech
MPGOBLE - Writes 'Brian L. Goble is a programming god!'
MPSCORPIO - Writes 'Brian L. Goble is a programming god!'
MPTOTARO - Writes 'Jim Totaro is da man!'
MPSUPERZUG - ? (if you know e-mail Coonz)
MPGOSHOPPING - Gives all items
MPNICENURSE - Gives +25 health
MPREALLYNICENURSE - Gaves 300 health
MPWARD - Gives ward (25 armor)
MPNEWCROWARD - Gives newcroward (100 armor)
MPCARBONFIBER - Gives Willpower Powerup
MPTAKEOFFSHOES - Makes you invisible
MPHERKERMUR - Gives Triple Damage Powerup
MPBEANSOFCOOLNESS - Gives a nice selection of weapons