From Sonic Retro
The Konami Code is an extremely popular Cheat Code, originally used in the Nintendo Entertainment System version of Gradius. Since it's original use, it's been put in hundreds of other games, (such as the Contra series) and referenced in movies and TV shows everywhere.
Usage in Sonic Games
Knuckles in Sonic 2 (prototype 0608)- Accesses the Level Select Screen, modified so the is not present in the code.
Sonic Chaos - Accesses the Level Select Screen, modified slightly so the and are reversed. Entering the code backwards will unlock the "Cheat Mode" and Sound Test.