I guess prototype 2650 S100 machines / cards qualify? There must be a lot
of homebrew "single instance" machines lying around. Here's one I'm
particularly fond of...
http://www.comcen.com.au/~adavie/weird/s100.html
This machine ran CP/M on a 2650 using a software 8080 emulator written in
2650. Slowly!
Cheers
A