Sonic the Hedgehog 3 and Sonic & Knuckles were originally to be released as one game,[4] but due to time and financial constraints, it was later split into two separate games. However, Sonic & Knuckles contained "Lock-on Technology", where the Sonic & Knuckles cartridge would be put in the Sega Genesis system, and then another game could be placed onto the top of the Sonic & Knuckles cartridge itself.[4] When Sonic 3 is used in this manner, it enables the player to play through both games as one, titled in-game as Sonic 3 & Knuckles. This features several additions to the games not available otherwise, such as small alterations to the levels, and being able to play through Sonic 3 levels as Knuckles, or Sonic & Knuckles levels as Tails, and the ability to save progress in all of the game's levels.Additionally, this was the only way to collect Super Emeralds, which are earned by accessing Special Stages in the Sonic & Knuckles levels after collecting all seven Chaos Emeralds from Sonic 3. When all Super Emeralds have been collected, Sonic, Knuckles and Tails can transform into Hyper Sonic, Hyper Knuckles and Super Tails respectively, each with unique abilities.

It's a Sonic the Hedgehog hack that includes Tails as a playable character, new/different layouts for every stage, different music ported from other Sega games, double boss fights in some zones and new moves!

Sonic And The Mystic Gems (previously known as Sonic 1 Harder Levels) is a hack made by Eduardo Knuckles and his team Harder Project of Sonic the Hedgehog on the Sega Mega Drive. It began in 2005 with the simple purpose of being a generic Sonic 1 for Sega Genesis with increase difficulty and revamped versions of the original levels. But now, it's going to present a completely new gaming experience with a new style of level layouts full of paths everywhere among new enemies, new objects, new character abilities, new menus, completely different styles of the Sonic 1 special stages and much more!Credits and Special ThanksMistergambit for his Spring Yard Zone and Labyrinth Zone level art used in the game.Tweaker for his information regarding music importing, bug fixes and the speedcap removal.HPZMan for the new sound driver instruments and the dynamic tile-set system.Selbi for some new exclusive ASM things and improvements.Master3k for sending and fixing some music.nineko for creating the xm3smps tool which was essential for the creation of some songs. Special credits to him for the different songs per act system used.SCAA for their art.Puto for his tutorial to add Spin Dash.FraGag for his tutorial to fix the spike bug.SonicVaan for beta testing.HCKTROX for the lead programming and bug fixing.GenesisFan64 for the save screen menu, for some fixes related to the fade-out and others.MarkeyJester for lots of important code and a whole load of others.Irixion for beta testing.PsychoSk8r for his the dynamic palettes tutorial.Esrael for fixing essential things in the new sound driver.Prilix Bõkun for bug searching and beta-testingEstlib for various art stuff related to levels and for additional help with creation of gimmicks and objectsDJ. Joker27 for his very big importance in specific level and misc musicMighty Arnold for additional support with beta-testing and level layout designKenny0989 for additional support in the music department Special credits to Harder Project's Team by all the useful exclusively things used in the project.

Sonic and the Secret Extended Edition (previously known as Sonic 1: The Other Extended Edition, not to be confused with Sonic the Hedgehog Extended Edition) is a hack of Sonic the Hedgehog for the Sega Mega Drive/Genesis by Hayate. It was discontinued in August 2008, after Hayate gave up on the hack due to the various bugs it had, but it was picked up again in December 2009.

The idea of this game is that you and another friend of yours selects a character (one being Sonic and the other being Tails) and you both battle it out to try to gain as many hits on Eggman. The catch is that Eggman only gets destroyed when the timer runs down to 0 (unless it's a draw at that time, then, it will wait for another hit). The player with the most hits at the end of the timer, wins! You can also win if the other player dies before the timer runs out. The amount of wins are tallied up on the results screen where you can either retry or change settings.This hack made by redhotsonic

Sonic CaDa is a hack of Sonic the Hedgehog by CamManS5.This hack features, new music, new art, secret paths, and multiple end points in each level that offer different rewards. The Special Stages have been changed into Bonus Stages. There will be new Special Stages based on the 2D Special Stages found in Sonic Triple Trouble. Since the new Special Stages are not implemented yet, the Chaos Emeralds cannot be collected at this time without cheating.