A couple of small OSs which might be useful for homebrew machines or homebrew CPUs:

http://retrobsd.org/wiki/doku.php
Small resource requirements. RetroBSD needs only 128 kbytes of RAM to be up and running user applications.
Real multitasking. Standard POSIX API is implemented (fork, exec, wait4 etc).

https://github.com/EtchedPixels/FUZIX
"FUZIX is a fusion of various elements from the assorted UZI forks and branches beaten together into some kind of semi-coherent platform and then extended from V7 to somewhere in the SYS3 to SYS5.x world with bits of POSIX thrown in for good measure."

Wow...it's been a long time since I heard anything about UZI. Thanks for both links; I'll have to check them out.