Oddest cheat codes or easter eggs you remember

Shanghai on Game Boy: Enter ZAP as a name, and you get a lighting sound effect (hence the name ZAP) every time you click on a tile that's already highlighted or illegal, or anywhere that's not a tile on the board.

 

Having to use cheat codes on the 3DO Game Guru in order to make use of the damned thing! Thanks JP for the tip about the Black Hole Assault code.