Mach

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

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 Hurd.
http://www.cs.cmu.edu/afs/cs.cmu.edu/project/mach/public/www/mach.html

Maruti
Goal: create an environment for developing and deploying critical applications with hard real-time constraints in a reactive environment. Such applications must be able to execute on a platform of distributed and heterogeneous resources, and operate continuously in the presence of faults.
http://www.cs.umd.edu/projects/maruti/

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/

MkLinux
An Open Source operating system which consists of an implementation of the Linux operating system hosted on the Mach microkernel.
http://www.mklinux.org/

Real-Time Mach
By Real-Time and Multimedia Laboratory, Department of Computer Science, Carnegie Mellon University. RT-Mach is a resource kernel, a kernel provides resource-centric services, which can then be used to satisfy end-to-end QoS needs.
http://www.cs.cmu.edu/afs/cs/project/art-6/www/

Real-Time Mach NTT
Another version of Real-Time Mach, by NTT Laboratories. Contains RT-Mach kernel, FreeBSD 2.2.8 server, real-time shell, RT-Java, Hurd loader. Getting Started section: readme, release notes, installation, hardware, compatibility, troubles questions and answers, and contacts.
http://info.isl.ntt.co.jp/rtmach/