Sonic CD Build 806 is a near final prototype of Sonic the Hedgehog CD for the Mega CD. Unlike other prototypes the disc comes complete with cover art and was released in stores as a demo CD in Japan. The demo has a limited amount of levels playable, but this can be circumvented with a level select code.
On August 31st, 2008, a dump was released by drx via torrent.
Some info taken from SonicRetro.
