All about the unreleased Sonic Xtreme Saturn and PC game.

http://www.sonic-xtreme.com/

pics/original music and more.

also from Wikipedia:
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A disc of a test engine of X-treme exists. A copy was sold at auction to an anonymous collector in September 2005, and a high-quality gameplay video was expected to be released by the end of the year. As of now, only an animated GIF image of the gameplay has been released to the community.

and

A second, more complete build (though still lacking a soundtrack of any kind) surfaced in February 2006 and was also sold into the hands of an anonymous collector. No footage of this version has been released, but it is suspected that the copy is actually a late build of the PC version. As there was at least one build for PC which did have several levels 'complete', this is plausible.
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