Press "T", type the cheat and hit "ENTER".

MPWHEREAMI: displays map coordinates
MPBEEFCAKE: more power for the character you play (also more gore)
MPKILLEMALL: kills all monsters on a level
MPSPEEDUP: increases the speed rating of your character (from 1 to 5)
MPSTRONGER: increases the strenght rating of your character from 1 to 5)
MPBERETTA: get the Beretta
MPSUBMACHINEGUN: get the submachinegun
MPFLAREGUN: get the flaregun
MPSHOTGUN: get the shotgun
MPSNIPERRIFLE: get the sniper rifle
MPHOWITZER: get the howitzer
MPNAPALMCANNON: get the napalm cannon
MPSINGULARITY: get the singularity generator
MPASSAULTRIFLE: get the M16/MP5 assault rifle
MPBUGBUSTER: get the bug buster spray can
MPMINIGUN: get the super chaingun
MPLASERRIFLE: get the ricocheting laser rifle
MPTESLACANNON: get the Tesla cannon
MPVOODOO: get the voodoo doll (if you simply hold it motionless for 30 seconds, it talks)
MPTHEORB: get the Orb
MPLIFELEECH: get the life leech (if you simply hold it motionless for 30 seconds, it talks)
MPGOSHOPPING: get all items
MPNICENURSE: +25 health
MPREALLYNICENURSE: +300 health
MPWARD: get 25% armor
MPNECROWARD: get +100 armor
MPCARBONFIBER: get the Willpower power up (95% invulnerable)
MPTAKEOFFSHOES: get the Stealth powerup (30 seconds of invisibility)
MPHERKERMUR: get the Anger powerup (30 seconds of triple damage)
MPBEANSOFCOOLNESS: get assorted weapons
MPFOLLOWME: play in 3rd-person mode (only works after you've upgraded to v.1.1)