As long as we're mentioning failed C= prototypes, don't forget the CDTV2, A1000+, and AA3000. Prototype machines are out there. Also the ill-fated A2200 promised by Computer Answers (it was a CD32 in a desktop case with some added boards).
How also about the PAL-version VideoToaster?
Phase 5 pre\Box?
Apollo ViperPPC
Individual Computers BuddhaFlash A500 version.