Type in the following codes during play:

MPCASPERInvisibility
MPVADERInvincibility
MPPLAYALLDAYInfinite Lives Mode
MPBUNZMakes Claw Strong
MPFRANKLINLightning Sword
MPPENGUINIce Sword
MPAPPLEFull Health
MPGANDALFFull Magic
MPFREAKCatnip
MPKFAGod Mode
MPLOADEDFull Ammo
MPSTOPWATCHTimer on the bottom of the screen
MPPOSShows global position
MPFPSFrames per second
MPOBJECTSShows # of objects on screen
MPNOINFOshow/hide info for above 4 cheats
MPLITHMonolith logos & Music
MPMONOLITHSame as above 
MPLOGOSame as above
MPMOONGOODIESBouncing treasure
MPSPOOKYInvisible to enemies
MPHAUNTEDEnemies into ghosts
MPSHADOWClaw is a shadow
MPSUPERTHROWSuper Throw mode
MPHOTSTUFFFire Sword
MPWIMPYClaw is weak
MPJORDANMega-jumping
MPBOTLESSToggles the background 
MPMIDLESSToggles the middle plane 
MPTOPLESSToggles the foreground
MPINCVIDIncreases resolution
MPDECVIDDecreases resolution
MPDEFVIDDefault resolution
MPJAZZY320X200 resolution
MPDEVHEADSShows developers heads in place of treasure
MPBLASTERFull dynamite

Thanks to Revolution readers Jan Wester, Dogus, and German Jakob!