Construction / Debug Mode (Sonic 3 & Knuckles)
To activate Debug, siply do the Level Select code in both Angel Island and Mushroom Hill.
Debug is not quite the same as it was in the previous Sonic games. You still press B to use Debug, A to place items, and C to cycle between items. However, when Debug isn't being use, the game timer is not messed up, and pressing A will cause gravity to reverse like it does in Death Egg Zone Act 2.

Level Select / Sound Test (Sonic 3)
When the Sega logo comes up, just after the "Sega" theme is sung and the screen begins to turn black, quickly press: Up, Up, Down, Down, Up, Up, Up, Up to the beat of the drums. You should hear a ring chime if you did it correctly. A new Sound Test option should appear at the title screen.
The Flying Battery, Mushroom Hill, and Sandopolis Zones are visible on the Level Select screen, but can't be played without Lock-On. Those levels were originally intended for Sonic 3, but were pulled in order to release the game on time.

Construction / Debug Mode (Sonic 3)
Enter the Level Select code, then press A + Start to choose your starting level. Once the action starts, press B to change into an item, A to cycle between the available items, and C to place an item.
Alternatively, turn on the Genesis and press Up, Up, Down, Down, Up, Up Up before Sonic gets fully on screen.

Sonic 2 -> Sonic 3 Cheat (Sonic 3 - duh)
If you can't do the super-tough Level Select code for Sonic 3, there is an alternative.
(WARNING: I'm not sure if this code is healthy for your Genesis or not, but it is not representative of recommended use, so I strongly recommend that you do not use it. Keep trying the regular Level Select code, or just do the much easier Sonic 3 & Knuckles Level Select instead.)
Insert Sonic 2 and enter the Sonic 2 Level Select (play sounds 19, 65, 09, and 17). Return to the title screen, hold A and press Start. To transfer this to Sonic 3, hold A and carefully remove the cartridge without turning off the Genesis. Insert Sonic 3 and press Reset. Finally, release A and press Down twice at the main menu.

Easy Emeralds / Superness
To gain all the chaos emeralds, enter the Level Select code, then highlight "Special Stage" and press C + Start. After receiving the emerald, press A + Start and repeat the process until you have them all.
Alternatively, enter the Sound Test code and play sounds 2, 4, 5, 6 in order. If you did it correctly, the Emerald sound will play.

Nightime Play
Enter the Level Select code, then press C + Start to choose your starting level.

The Missing Special Stage
Access a special stage not found with Sonic and Knuckles!!!!!! For an all new bonus stage, enable Level Select, then play tune 1F. Finally, highlight stage 2 and hold A while pressing Start.

Bonus Boards Unlimited
Enter the Level Select code, then highlight the "2P Vs" option and press START. You will enter a series of bonus stages that last until you reset the system or press A+START.

Special Controls - Escape, Slow Motion, Frame Advance
After the Level Select / Sound Test Code has been entered, pause in any level. Press A to escape to the game's title screen, B to advance several frames ahead in time (repeatedly for slow-motion), or C to advance one frame ahead in time.

Animation Gallery
After the Construction / Debug Mode Code has been entered, enter a level, and press B while holding C. Your character will start cycling very quickly through every animation he has. Use the Pause-C "frame skip" to see every frame of every animation. There are several that were not used in the final game, including Sonic whistling and Sonic falling asleep.

Unlimited Rings and Unlimited Super/Hyper
Enter Debug Mode then select a stage. Choose the monitor as your item. Place one (Ity will have an "S" on it) and collect it to get 50 rings and become Super. You can do this as many times as you want, meaning that you have unlimited rings and can remain Super as long as you want.

Extra Lives At The Launch Base
To do this trick you must get to the first level of the Launch Base Zone. Set off the first alarm, stop Sonic in the middle of it, and do a Spin-Dash attack. The flybots will continue to attack, and eventually their value will increase to 10,000 points! Every five flybots will gain you a life and, if you're patient enough, you can earn up to 99 lives!

Extra Rings For Tails
Enter Debug Mode then select a stage. Next press B, A (you must have Sonic and Tails on-screen.) When your cursor changes into a ring, paste it on Tails.

Super Shortcut In The Balloon Park
Go to Competition and select Balloon Park. Instead of going forward, fall down backward to land on a yellow balloon. Hold LEFT, You should walk on the ceiling. You will disappear and keep going until you win!

Turbo Tails !!!!! (Sonic 3 Only)
Enable Level Select, then press Down to make sure you have the sound test (don't select it yet). Start a one-player game and choose Tails in the Data Select screen. When the timer appears, press RESET.

Enter the Sound Test menu and choose any zone. Hold A and press Start. You should be Tails and be in debug mode. From here press B, then press A until you see a item monitor. Press C to select it, then press B to return to Tails.

To enable Turbo Tails, jump on the box. Tails will look weird and can not be hurt or lose any rings. He can still get squashed and drowned, though, so be careful in the Ice Cap zone. You will have do this at the beginning of each zone. Robotnik can hurt him at the very end of Launch Base zone-act 2.

Changing Identities
Use the debug mode cheat. Start any level that has a special stage ring. Next turn into your favorite object. As an object go to the center of a ring. Now hit B twice quickly. When you get out of the special stage you will look like that object for the rest of the level.

Battle Music In the Hydrocity Zone
Just before Before you fight Robotnik in the Hydrocity Zone, act 2, the boss music will come. Keep Sonic's head under the shallow water until he is about to drown, then pop out of the water. You will hear the boss music from Sonic and Knuckles.

Mecha Sonic and Mecha Tails
Use the Debug Mode code in Launch Base act 2. Beat the game. When Sonic or Tails jumps towards you, activate Debug Mode and get out of it immediately. You will be able to use Sonic or Tails in the background for a short time, but they will look like robots.

Black and Blue Knuckles
In debug get to Carnival Night act 2. Get to where knuckles hits the lever and activates the anti-gravity tubes. Get in and out of debug quickly. As soon as Knux hits the lever and starts to run, go after him and outrun him. Go back to where he was, he should have a odd blue body and a black face. If he isn't there go back to the tube and he'll reappear. Repeat the process.

Random Start
Enter the sound test menu choose a level. Look for a star post, touch it and hold A and press START. Then choose another level. If done correctly the title of the zone will change. Beware you might start out in a wall so use debug mode to fly out.

Super Super Zone Shortcut
In debug go to any zone except sandopolis. Change into a sprite and go as far back as possible. Get out of debug. You should start flying super fast (Get back into debug because who knows where you'll end up); Just enjoy the ride.

Fake Act
(This does not work in Ice Cap 1 but it will send you to act 2 and won't work in Mushroom Hill, Flying Battery, Sandopolis, Lava Reef, Hidden Palace, Sky Sanctuary, or Angel Island)
Go to the bottom of the screen in debug. You should see a ceiling the background, and some black (in some zones you may have to travel under near the boss area). Go right to the end of the screen and press up until you enter a small replica of the act (to get out use debug and try to move left at all times.)

Game Genie
(Sonic 3 Only)
AA6T-AAXC Level select
AWHA-CA92 Don't lose rings
SCGT-DJYL Rings worth 2
SCGT-DNYL Rings worth 3
SCGT-DTYL Rings worth 4
SCGT-DYYL Rings worth 5
SCGT-D2YL Rings worth 6
SCGT-D6YL Rings worth 7
SCGT-DAYL Rings worth 8
KRGT-CAE0 75 Rings needed for a free guy
GLGT-CAE0 50 Rings needed for a free guy
D4GT-CAE0 30 Rings needed for a free guy
CWGT-CAE0 20 Rings needed for a free guy
BLGT-CAE0 10 Rings needed for a free guy
HCGT-DJZY Get 2 free guys for 100 rings instead of 1
HCGT-DNZY Get 3 free guys for 100 rings
HCGT-DTZY Get 4 free guys for 100 rings
HCGT-DYZY Get 5 free guys for 100 rings
HCGT-D2ZY Get 6 free guys for 100 rings
HCGT-D6ZY Get 7 free guys for 100 rings
HCGT-DAZY Get 8 free guys for 100 rings
AC5A-CAHN Super ring powerup worth 0
A05A-CAHN Super ring powerup worth 5 instead of 10
B85A-CAHN Super ring powerup worth 15
CW5A-CAHN Super ring powerup worth 20
DG5A-CAHN Super ring powerup worth 25
D45A-CAHN Super ring powerup worth 30
GL5A-CAHN Super ring powerup worth 50
NW5A-CAHN Super ring powerup worth 100 (1-Up every time
AJZA-CA8N Super Sonic doesn't lose rings
AJ3A-CA7A Infinite lives
AG4T-AAH4 Start with 1 life
AL4T-AAH4 Start with 2 lives
A04T-AAH4 Start with 5 lives
BL4T-AAH4 Start with 10 lives
DG4T-AAH4 Start with 25 lives
GL4T-AAH4 Start with 50 lives
NR4T-AAH4 Start with 99 lives
ACAT-CAB6 Shield can be activated multiple times
AJRT-CA8R Invincibility power-ups last forever
986A-CAB6 Invincibility lasts almost twice as long
KR6A-CAB6 Invincibility lasts half as long
E06A-CAB6 Invincibility lasts 1/4 as long
SCFT-BJT4 Only need 6 chaos emeralds
SCFT-BNT4 Only need 4 chaos emeralds
SCFT-BTT4 Only need 3 chaos emeralds
SCFT-B2T4 Only need 2 chaos emeralds
SCGA-BL0N + A4GA-A28T Blue spheres worth 2
SCGA-BR0N + A4GA-A28T Blue spheres worth 3
SCGA-BW0N + A4GA-A28T Blue spheres worth 4
SCGA-B00N + A4GA-A28T Blue spheres worth 5
SCGA-B40N + A4GA-A28T Blue spheres worth 6
SCGA-B80N + A4GA-A28T Blue spheres worth 7
SCGA-BC0N + A4GA-A28T Blue spheres worth 8
ACGA-AA8T 1 Blue sphere needed for chaos emerald
TAYA-CRB6 Bonus jump
AAYA-CWB6 Super jump
AAYA-C0B6 Mega-jump
ATST-CA38 Never drown (must switch off for some spring-pad jumps)