Type in during gameplay:

Code - Result

JOSHUA - Godmode.
HIGHLANDER - Never lose a Life.
QUICKEN - Put a "Q" in gamefield.
VANGOGH - Puts a Gun in gamefield.
CHAINMAIL - Puts a Sheild in gamefield.
PINEAPPLE - Puts a Gernade in gamefield.
JOSHUA - Godmode.
DIGDUG123 - Gives you a "Debug" menu option.