Remember . . . if it has an asterisk (*) beside it, that means I have personally checked that code out and it works. If there is no asterisk, then it is a code that I have neither checked out nor disproved. You're on your own with them.
Control Mode
Code - At the title screen, press Up, C, Down, C, Left, C,
Right.
Affect - During gameplay, when you press A, the game will
restart. Pressing B will put the game in slow motion, and C will advance
a frame.
Debug
Code - Enter Control Mode. Press Start to begin the game,
then hold A until Sonic appears on the screen. If Hex numbers appear at
the top of the screen, you're in.
Affects - During gameplay, B will turn Sonic into another
sprite, A will change the sprite, and C will place it on the screen. Each
level has its own sprites.
Note - This will only work on first production releases of
Sonic the Hedgehog.
Level Select
Code - At the title screen, hold A and press Up, Down, Left,
Right. When you hear a chime, press Start.
Affects - You enter the Level Select screen.
Emeralds & Continues.
Code - Enter Level Select and go to the Special Stage. Get
the emerald, then you will enter the first stage. Hit Reset (do NOT turn
off the power completely), then repeat the process.
Affects - Every time you enter the Special Stage and then
Reset, you will always start off with the number of emeralds you have
previously collected, as well as the continues you have earned by
completing the stage. After you have all the emeralds, just start up a
regular game and you will still have them all.