HURD
The GNU HURD is the GNU project's replacement for the Unix OS kernel. It is built on top of GNU Mach4. The project is still going on now, and they join Debian (Linux) team to make an easy-to-install version of it.Running on top of Mach should make it possible to run on multi-processors environments. Also it should give more flexibility to the system.
This page is linked from: Mach Microkernel Microkernel Debate