What cheat codes have remained in your memory to this day?

[51JBerger]

The Konami 30 Life Code: Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.

The Sega Genesis Mortal Kombat Blood Code: A, B, A, C, A, B, B.

This one possibly may not be a cheat code, but I remember the Mew Glitch in Pokemon Red, Blue, Yellow.