The unfinished Sega Saturn era Sonic the Hedgehog title, Sonic X-Treme, has been released to the world by a group of fans.
The game is more of a tech demo than anything else as Sonic X-Treme never reached a state of completion, but fan group SonicRetro has polished the source code that was available and released it as a playable build.
The release only includes a single stage that was initially shown during an E3 promotional reel back in 1996. SonicRetro intends to add more to the project as it works on compiling other files that it has recovered.
In rebuilding the game for modern machines, the fan group had to make several adjustments to the engine and essentially port the the whole thing over to modern Windows and Open GL. The group has ported the engine as close to how the original developers had left it, describing it as 'a snapshot in time'
The Sonic X-Treme tech demo can be downloaded from the SonicRetro forums
The Sonic X-Treme project died by falling between two consoles. Initially scheduled for a 1996 release, the title started life as a 2D platformer on the Mega Drive but was then adapted half way through into a 3D title for the Sega Saturn, resulting in several development team and engine reshuffles. The game was officially cancelled in 1997.
Following the cancellation, this left the Saturn without a true Sonic game for the system and Sonic's true 3D debut ended up being in Sonic Adventure for the Dreamcast in 1998.