When you get to the purple dragon, type "DIE" and he will drop a purple key. Take the key to Marvo and he will give you a cheat mode in return.
Typing "BLOOD" makes Mortal Kombat-style blood fly out of your enemies.
Typing "SUN" makes the sun wink at you.
Typing "WHAT A CROC" makes Dibble shout, when you do something dumb.