Terracide

(Enter under the Help Menu option in 
the Cheat Codes password box): 

barnsley = Invincible mode on/off
nozzle = Access to all episodes and levels through
         the Game-Levels Menu option
popov = Go anywhere mode on/off
        clam_chariot = Loads of cheats (including all guns)
welk = All guns available
phaal = Unlimited ammo on/off
batfinc = Full shields
drugssuck = Numbers displayed on screen on/off
top_trumps = "Go anywhere mode on" message displayed,
             but it doesn't work
yams = "Go anywhere mode on" message displayed,
        but it doesn't work