Most of the Sonic the Hedgehog 2 Codes that you can unlock/perform. These are the well known codes.

Code - Effect
14 Continues - Options Sound Test: 01, 01, 02, 04 in that order.
From Tails to Miles - Press up, up, up, down, down, down, up at the title screen
Level Select - Sound Test: 19, 65, 09, 17 (In that Order), Exit and Hold A at the Title Screen, then Press Start.
Debug Mode - While in Level Select: In the Sound Test, press 01, 09, 09, 02, 01, 01, 02, 04 in that Order. Hold A and choose a Stage.
Super Sonic early in the game - While in Level Select: In the Sound Test, press 04, 01, 02 and 06 in that order.