Sonic & Knuckles (ソニック&ナックルズ Sonikku to Nakkuruzu?) is a 1994 platform video game in the Sonic the Hedgehog series for the Sega Genesis. It was developed in the United States at Sega Technical Institute by members of Sonic Team, and was published by Sega worldwide on October 18, 1994.In addition to being a standalone game, through a system dubbed "lock-on technology," Sonic & Knuckles acts as an expansion pack to both Sonic the Hedgehog 2 and Sonic the Hedgehog 3, the latter with which it was developed in tandem.

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 Classic Heroes is a ROM hack for the Sega Mega Drive / Genesis that is the result of a collaboration between flamewing and ColinC10, combining their respective hacks Sonic 2 Heroes and Sonic 1 and 2. It therefore is a single ROM that offers the entire campaigns of Sonic the Hedgehog and Sonic the Hedgehog 2, either alone or together; and enables the player to play as many characters such as Sonic, Tails, and Knuckles. The hack is named due to the ability to control a team of characters, which is reminiscent of the more recent 3D game Sonic Heroes. The hack is built on Sonic 2's engine, which has been enhanced by both flamewing and ColinC10 to have new features. These include: multiple gameplay elements from Sonic 3 & Knuckles, such as element shields and character-specific abilities; the ability to save progress, scores, etc. to SRAM (something rarely implemented in Sonic 2, with the other notable example being Esrael's Sonic 2 Delta); various bug-fixes and improvements to the mechanics of the original games; and many more features, both implemented and planned.

"Sonic G6 Lite" a hack of sonic 1 that is speed and difficulty based and takes place at night time setting .The hack made by joshwoakesm