Sonic the Hedgehog
The Sega Genesis had "add-on" systems that incorporated "Sonic" games. "Sonic CD", released for the Sega CD, was a 2D platform game released in 1993. The game introduced the characters Amy Rose and Metal Sonic and featured levels that differed depending upon whether Sonic was in the past, present, or future time frames. "Knuckles' Chaotix", a spin-off released in 1995 for the Sega 32X, featured Knuckles and a new group named Chaotix fighting against Dr. Robotnik. The game featured a two-player cooperative system in which the on-screen characters were connected by magic rings. There was also "Sonic Eraser", a puzzle game released exclusively on Sega Game Toshokan in 1991.
Sega Forever
The initial wave of games were from Sega's Genesis/Mega Drive, Game Gear, and Master System platforms, although they plan on adding games from the SG-1000, Sega Saturn, and Dreamcast platforms as well. During the testing phases of the system, Saturn and Dreamcast games did not perform satisfactorily, though Sega has on-going R&D efforts working on improving them in hopes of future release. Games such as "Panzer Dragoon" are being considered for the service, but could not be successfully emulated in the testing phase, meaning that such a release would hypothetically require a lengthy porting process instead. Sega is also open to releasing games that had not previously had English localizations, such as the Yuji Naka-designed game, "Girl's Garden".