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 disproven. You're on your own with them.
*Control Mode
Code - Level Select, start a level, then pause anytime during
gameplay.
Affect - While paused, pressing A will cause the game to
reset. Pressing then holding B will make the game go in slow motion.
Pressing C will advance the game one frame.
*Debug
Code - Level Select, then play sounds 1, 9, 9, 2, 1, 1, 2,
then 4 in the Sound Test. You will hear a ring go "ding!" if you did it
right. Highlight any level, press and hold A, and press start. If Hex
numbers appear at the top of the screen, you're in.
Affects - Press B to turn Sonic into another sprite (aka to
start debugging). Pressing A will change the sprite, and pressing C will
place the sprite. Press B again to turn back into Sonic.
*Super Sonic
Code - Level Select, then play sounds 4, 1, 2, then 6. You
will hear the music that you normally hear when you get a Chaos Emerald.
Pick a stage and go.
Affects - After you grab fifty rings, you can become Super
Sonic without any Chaos Emeralds!
*More Continues
Code - Enable Level Select, but DO NOT ENTER IT! Instead, go
to the Options screen. There, play sounds 1, 1, 2, then 4. Then go up to
the Player Select box and press Start to begin a game.
Affects - The upside? You get 14 continues! The downside?
You have to listen to the Chemical Plant Zone music throughout the ENTIRE
GAME! ARGH! As if I wasn't batty enough already! Well, there is one
more notable effect. If you hit the Restart button, the blasted CPZ music
stops, and you get treated to one or many rings going "ding!".
*Night Play
Code - Level Select. Highlight the level you want, press and
hold C, then press Start.
Affects - It won't actually look like nighttime, but certain
parts of the screen will become very dark. Not very helpful, just weird
to look at.
*Miles/Tails
Code - At the Title Screen, press Up, Up, Up, Down, Down,
Down, then Up. You will hear a ring go "ding!" if you did it right.
Affects - Everywhere it says "Tails" will now say "Miles", and
vice versa. Eye candy.
*Two, count 'em, TWO Tails!
Code - Level Select and Debug to the Sky Chase Zone (or you
can play through the game to it, but this is quicker). Start debugging
and make a transfer box (it's the second sprite on the list). Quit
debugging and break open the transfer box and wait until you hear Sonic
die. When you hear it, press B again to start debugging. Get a little
above the bottom of the screen and stop debugging again. You may have to
jump to make the picture come back.
Affects - Yes, the code SOUNDS tough, I know, but believe me,
it isn't really all that hard, and it's definitely worth it just to see
this! When the you reappear on the screen, there will be two (count 'em,
two) Tails flying two (count 'em, two) airplanes! Great eye candy!