Enter the following at the 'Cheats' Screen:

GOLDENPIE   Enable all cheats
GODLY       God mode
LOADED      All weapons 
SKETCHY     Pen and Ink mode
SKIPPY      Level select
SONIC       Faster running
SORTED      Infinite ammo

CLAW        Egypt Fodder in multi-player
GODDESS     Female trooper in multi-player
LEGGY       Hive guard in multi-player 
RUBBER      Volcano guard in multi-player
UGLY        Hive Fodder in multi-player  

Thanks to Revolution readers Nick Noel, Nate Norton, I\Ia'I'ha/\/ §., Josh Wagner, Sean Stubsten, BISHOP, Tim Sandner, Michael F., David, Andrew Vidal, jeff, Mac, carnage587, Walter Dailey, Fizban, and Luis Gaud!