Sonic The Hedgehog

Level select:
Press Up, Down, Left, Right at the title screen. A chime will confirm correct code entry. Hold A + Start to begin game play.

All Chaos Emeralds:
Enable the "Level select" code. Select the "Bonus level" option on the level selection screen. Obtain the Chaos Emerald, reset the game, and repeat the process until all of them are in your possession. Allow the game to continue after obtaining the last Chaos Emerald to start in the Green Hill Zone.

Debug mode:
Press Up, C, Down, C, Left, C, Right, C, Start, then hold A at the title screen. Continue to hold A until game play begins. Screen coordinates will appear instead of a score and the number of objects displayed will appear instead of the time remaining. Use one of the following controller actions to enable the corresponding debug code. Note: This code may only be enabled on the first version of the game.

Object transformation
Press B during game play to change Sonic into a game object. Press A to change into a new object. Press C to place an object on the screen. Press B again to revert back to Sonic. Note: The type of object that may be selectable will vary from level to level.

Invincibility
Sonic is, by default, invincible in debug mode.

Slow motion
Pause game play and hold B.

Restart
Pause game play and press A.

Frame advance
Pause game play and press C.

Crazy Sonic:
Hold A + B + C during demonstration mode for force Sonic to run into objects and act strangely. Note: This may also be done during the ending sequence after completing the Final Zone.

Bonus points:
Complete Green Hill Zone 1-1 or 1-2 in 29 seconds or less to receive 50,000 bonus points.

Unlimited lives:
Collect the extra life at the very bottom of Green Hill Zone - Act 3. Accumulate 100 rings to obtain another extra life. Collect a third extra life located at the top of the loop-the-loop. Intentionally lose a life and repeat the process.

Cheat Codes:
U, D, L, R, A+Start - This enables level select.
U, C, D, C, L, C, R, C, A+Start - This is Debug Mode.
(C, A, C, B, U, D, L, R, A+Start - The alternative)