Sonic X-Treme
Sonic X-Treme was a title that was going to be released for the SEGA Saturn and developed by the now-defunct SEGA Technical Institute. Due to extreme time constraints and SEGA of Japan consistantly ordering STI to re-develop the game from scratch, the game never saw the light of day.

Sonic X-Treme was a revolutionary title. It incorperated the mind-bending, speherical presentation into a true, 3D Sonic experience. The game allowed Sonic to rotate the world around him, allowing him to discover new routes and secrets. It also incorperated a fish-eye camera to give the already vomit-inducing stages a more speherical look.

More recently, a disc containing the last known playable version of Sonic X-Treme came up for auction on the internet. The winning buyer managed to rip the game into a digital format so it can be played on an emulator.

Thanks to Chris Senn (the game's designer), Sonic fans now have a complete source for everything Sonic X-Treme. the 'Sonic X-Treme Compendium' is run by Chris Senn and houses all the remnants of Sonic X-Treme that Senn still holds.
Screenshots

Click the thumbnails for larger images.
Video Footage
Early Gameplay Trailer - A very early gameplay trailer that features gameplay footage from a few stages as well as a boss battle.
Red Sands Gameplay - Gameplay footage of the Red Sands stage.
Sonic X-Treme: Final Version Gameplay - Gameplay footage from the disc containing the last known playable version of Sonic X-Treme.
Death Egg Gameplay - Gameplay footage from the Death Egg stage.
Jade Gully Gameplay - Gameplay footage from the Jade Gully stage.
Sonic X-Treme Pitch Demo - Demo footage used to pitch the original concept that would become Sonic X-Treme.
Crystal Frost Gameplay - Gameplay footage of the Crystal Frost stage.