Sonic X-Treme Playable Build Released
After 19 years, a programmer by the name of JohnnyBravo found a build of the engine, and since then has been working to make Sonic X-Treme playable on your PC. You can download the playable build here.
If you want to know more about Sonic X-Treme, miiyouandmii2 has done a 3-part documentary on the game.