<A HREF="http://www.cs.vu.nl/~ast/minix.html">Minix</a> is _(Andy Tanenbaum)'s famous *(Unix)-like *(educational) *(OS) in 2 versions:

<UL>
  <LI>MINIX 1.5 (Intel, Macintosh, Amiga, Atari, SPARC)</li>
  <LI>MINIX 2.0 (Intel CPUs from 8088 to Pentium)</li>
</ul>

But if you're looking for a "power" Minix try out 
<A HREF="http://www.minix-vmd.org/">Minix-VMD</a> it does support X11 and Virtual Memory (and much more of course !)