Minix
Minix is Andy Tanenbaum's famous Unix-like educational OS in 2 versions:- MINIX 1.5 (Intel, Macintosh, Amiga, Atari, SPARC)
- MINIX 2.0 (Intel CPUs from 8088 to Pentium)
But if you're looking for a "power" Minix try out Minix-VMD it does support X11 and Virtual Memory (and much more of course !)
This page is linked from: Andy Tanenbaum Microkernel Microkernel Debate