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 start a level by holding A and
pressing Start while it is highlighted. 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.
*View Animations
Code - Level Select and Debug. Begin debugging, then press B
and C at the same time.
Affects - Sonic and/or Tails will begin to go through all of
their animations. Very freaky to watch. I highly recommend it!
*Super Sonic
Code - Level Select, then play sounds 2, 4, 5, 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!
*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.
*Extra Lives
Code - Play to the Launch Base Zone. Find one of the alarms
and stand in the center of it while it blares and lights up. Enter a spin
dash (Down and B), but don't release it. A Flybot will come by every few
seconds and run into you, destroying itself. Continue to hold the Spin
Dash and more Flybots will hit you, gradually increasing in pointage until
they are worth up to 10,000 points and an extra life!
Affects - You will keep getting extra lives until you get
bored or the counter decides not to count them anymore. Duh.
Secret Special Stage
Code - Level Select and play sound 1F. Highlight stage 2,
then hold A and press Start.
Affects - Accesses a Special Stage not accessable otherwise.