Fans have managed to wrestle together a tech demo for the cancelled 1996 Sega Saturn and PC title Sonic X-Treme that runs on modern Windows machines. You can find all details of the effort that went into it on SonicRetro, including a way to download it.
The current downloadable version only features a jungle level that was shown in a showreel back in 1996 at E3. It’s such an early build that you can’t even collect rings. You can get a look at Sonic’s gravity-defying spin mechanic, however. More levels are expected to be excavated by the fan team currently working with the original source code.
Sonic X-Treme would have been the blue hedgehog’s first foray into 3D had it been finished. It was being developed by Sega Technical Institute but due to many problems with the engine and key staff falling very ill due to long work hours, the game was cancelled a couple of months before it was due to be released.
The video below details some of the history of Sonic X-Treme and also goes into the work that fans have recently put into it to make it playable.