Press T while playing then type in one of the following:

MPGOD - god mode
MPKFA - full weapons
MPAMMO - full ammo
MPCLIP - no clipping
MPHEALTHY - full health
MPWHEREAMI - displays your coordinates
MPHIDEME - hides your coordinates
MPBEEFCAKE - increased power
MPKILLEMALL - kills all monsters on level
MPSPEEDUP - increases your speed (1-5)
MPSTRONGER - increases your strength (1-5)
MPCALEB - changes you to Caleb
MPOPHELIA - changes you to Ophelia
MPISHMAEL - changes you to Ishmael
MPGABBY - changes you to Gabriella
MPBERETTA - beretta, type twice for dual weapon
MPSUBMACHINEGUN - submachingun, type twice for dual weapon
MPFLAREGUN - flaregun, type twice for dual weapon
MPSHOTGUN - shotgun, type twice for dual weapon
MPSNIPERRIFLE - sniper rifle
MPHOWITZER - howitzer
MPNAPALMCANNON - napalm cannon
MPSINGULARITY - singularity generator
MPASSAULTRIFLE - assault rifle
MPBUGBUSTER - bug buster
MPMINIGUN - minigun
MPLASERRIFLE - cobalco laser rifle
MPTESLACANNON - tesla cannon
MPVOODOO - voodoo doll
MPTHEORB - the orb
MPLIFELEECH - life leech
MPGOSHOPPING - all items
MPNICENURSE - 25 health
MPREALLYNICENURSE - 300 health
MPWARD - ward (25 armor)
MPCARBONFIBER - willpower powerup
MPTAKEOFFSHOES - invisible
MPHERKERMUR - triple damage powerup