Referred to in-game as Knuckles the Echidna in Sonic the Hedgehog 2, the game is largely identical to Sonic the Hedgehog 2, with the exception that gameplay is altered due to Knuckles having separate abilities from Sonic or Tails. Knuckles can glide and climb walls, allowing him to gain access to areas that had been otherwise hidden or unreachable. Conversely, he cannot jump as high, making some situations, such as certain boss fights, especially the final one, more difficult. In this version of the game, the two player mode has been removed.

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.

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.

Sonic the Hedgehog hack made by By Narcologer fully remade zones: Paradise Coast (GHZ), Border Fort (MZ), Dangerous Pit (LZ) and Robotropolis (SYZ) with new music, layouts, graphics and some new chunks. characters: Sonic, Mighty and Metal Sonic. Sonic can perform spin dash and peel out, Mighty can perform jump dash and double jump, Metal Sonic can only to spin dash but also he can't drown.Because 3 characters (and there will be more in future releases!) are too many for title screen I made a simple character select menu.A lot of new sprites.New Sega logo screen.And lots more!Credits:Programming: Narcologer, Vladikcomper, Sonic2010, EsraelNew level art, new chunks and layouts: NarcologerSprites: Narcologer, The Spriters Resource, SCAA, Lone DevilMusic: Ivan YO, STD, Nineko

Sonic Chaos Quest v2.0 is a modification of Sonic the Hedgehog. Story:Exploring Mobius seas Eggman found the Chaos Island. It has giant lake, on bottom of which the sunken ruins of Chaos Shrine which contain the inexhaustible power of Chaos are lying. In order to increase the power of his mechanisms Eggman built the Hydrostation on this lake and plans to conquer the world with the power which he got. Residents of the island asked Sonic for help, so he teamed up with Mighty, Tikal and Metal Sonic to start their Chaos Quest to stop Eggman's evil plans once again!Credits:Programming: Narcologer, Vladikcomper, TheBladNew level art: Narcologer, ShooterNew chunks and layouts: NarcologerSprites: Narcologer, The Spriters Resource, SCAA, Lone DevilMusic: Ivan YO, EgorTC, Nineko, TheBlad

Sonic Double Dash is a hack based on sonic 1 that greatly revamps the first zone into something that you'll hopefully consider fresh and fun. Technical changes are relativly minimal as the hack has a greater focus on creating new layouts and artwork. The levels are built with the new double dash in mind, a redesigned drop dash that's power can change the flow of how you play, epscially useful for speedrunners or those who just want to have a good time. Forget that last part as it makes this sound like a dating profile.