Kyphus
Kyphus is an unreleased Atari 2600 prototype from Apollo, programmed by Tim Martin around 1982. It was never sold, and the only known copy of the prototype surfaced in 2006. What survives is an early work in progress: a strange pseudo-3D maze with very little actual game in it, but a rare look at a project that was lost for more than twenty years. You can try it here in your browser.