Sonic 1 Cheats (Genesis)

At the title screen, press Up, C, Down, C, Left, C, Right. While playing, you can pause
the
  game and use A to restart, B for slow motion, and C for frame advance.

Enable Control Mode. After pressing Start to begin a game, hold A until you see Sonic. If
you
  see a bunch of hex numbers on the top of the screen, you're in Debug Mode.

  To start debugging, press B, then you can select any sprite in that level and put it on
the screen.
  To select a different sprite, press A, and to place it on the screen, press C. Note
that sprite
  selection is different from Zone to Zone. Also, instead of displaying the time elapsed,
it displays
  the number of sprites currently on the screen. Control Mode functions work as well.

  Note: This only works on the first production releases of Sonic.

At the title screen press up c down c left c right.start the game.then,when you press
pause then a,the game will restart.press c the game will move one frame,press b,the game
will be in slow motion!

In Marble Zone act 3 you will find three chandeliers with spikes in them. Go to the far
wall (on
  the right) and jump to the right to find some goodies. Now jump through the wall on the
right
  again, but be careful of the other chandelier.

At the title screen, hold A then press Up, Down, Left, Right. When you hear the chime,
press
  Start. 

  If that doesn't work, try this: at the title screen, press Up, Down, Left, Right, hold
A, and press
  Start. After doing this once, you only need to hold Start + A to enter the level
select.

In Green Hill Zone - Act 3, there is a 1-Up at the very bottom of the level. Collect it
along with
  enough rings to make 100 for a second life. Next, grab the 1-Up at the top of the
loop-de-loop
  and die. Repeat until you have enough lives.

For extra points, pass any zone in a certain amount of time. For the best bonus, enter the
Debug
  Mode code. The clock will freeze and you'll always score a 50,000 bonus!

0:30 50,000
  0:45 10,000
  1:00 5,000
  1:30 4,000
  2:00 3,000
  3:00 2,000
  4:00 1,000
  5:00 500

This cheat allows you to start at the First Level with all the Emeralds, and with lots of
Continues.
  Use the Level Select cheat (see above) to get to the Special Zone. When you get the
Emerald,
  and have entered into the First Zone, press Reset.

  Use the Level Select cheat to get to the Special Zone again. When you get the next
Emerald, and
  have entered the First Zone, press Reset again.

  When you have all the Emeralds, continue on with the first level. You get to keep the
Emeralds
  and the Continues accumulated so far.

At title screen type in :up,A,,down, A,left, A ,right ,A then A,start. This is a special
level select,, it also allows you to hear the sounds and music.

On the last level (Scrap Brain), find the ledges that move right then left. Go to the left
side of the
  wall and wait for the platform to move. As soon as it starts to move left, duck. You'll
go through
  the wall. While here, press Left to teleport right and Right to teleport to the left.

While using the debug mode, enter any special stage. Turn into any object and travel
outside the
  special stage area. Ou will fall into a secret special stage filled with one-ups and
chaos emeralds.

While the demo is playing, hold A + B + C. Sonic will get confused, lose his place, and
possibly die.