Cheat mode: To unlock the level selection and sound test screens, at the title screen, press Up, Down, Left, Right. Then, hold the button that corresponds to A and press Start.
Debug mode: To unlock Debug mode, at the title screen, press Up, Circle, Down, Circle, Left, Circle, Right, Up, Circle, Down, Circle, Left, Circle, Right, Circle. Then, hold Square and press Start. Begin a game, then press X.
All Chaos Emeralds: At the title screen, press Up, Down, Left, Right. If you entered the code correctly, you will hear a chime. Hold Square and press Start. Choose your starting level. When you get to the main game, press Start then press Square to return to the title screen. When Sonic starts to appear, press Up, Circle, Down, Circle, Left, Circle, Right. If you entered the code correctly, you will hear a chime. Hold Square + Start to access the level selection screen. Highlight the Special Stage. Hold Square + Start when begin, and hold the buttons until game the starts. You can go into debug mode and float over to the Emerald. Repeat until you have them all. Then during the actual game, enable Debug mode and zip through them all. This is the easiest way to finish with all Chaos Emeralds.
Enable the "Cheat mode" 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.
Bonus points: To get 50,000 bonus points, complete Green Hill Zone 1-1 or 1-2 in under 29 seconds.
Unlimited lives: To get 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.
Special Zone entrance: You can almost always have the Special Zone entrance open (the giant ring next to the sign at the end of level Acts 1 and 2) by not losing your rings. If you do lose your rings, when you pass a checkpoint, intentionally die, then continue through the level without losing any rings. This works about 80% of the time.