Mach  
   Famous pioneering microkernel research OS, from Carnegie Mellon University: CMU. It lead to many well known OSs: NeXT, Flux (part of the Flux-Fluke-Flask progression, though Fluke is new code, and which is viewed by many as the follow-on to Mach), GNU Hur
   http://www.cs.cmu.edu/afs/cs.cmu.edu/project/mach/public/www/mach.html

Flux Research Group: Older Projects and Software  
   Mach 4 kernel, Lites Unix server, Mach/Lites/4.4-lite, MOSS, Goofie, PA-RISC GNU tools.
   http://www.cs.utah.edu/flux/oldproj.html

MK++  
   Mach compatible microkernel; gives one code base able to support high assurance, scalability, realtime, distribution, SMP, fault handling, performance; all built in an object-oriented B3 evaluable fashion.
   http://www.opengroup.org/RI/technologies/mk-dbleplus/

Yamit  
   Full POSIX, OSF Mach-derived microkernel with many concepts revised to fix Mach flaws. Scalable, portable, for tightly (SMP) and loosely ([cc]NUMA) coupled multiprocessor systems. Ports: x86, x86-64, IA-64; MIPS, PPC, HP-PA, ARM. [Open Source, BSD-like]
   http://yamit.sourceforge.net/