Mach
Mach is a free microkernel upon which many experimental OSes are built. There exists "ports" of classical Unix to Mach, BSD (Lites) and Linux (MkLinux), consistently slower than their "monolithic" counterparts.The original CMU Mach project is now ended with Mach3 (see Published and Unpublished Mach Papers). Some pretty interesting ones are available here. Two different groups are working on two divergent further versions of it:
The Open Group (formerly known as the Open Software Foundation (OSF)) is officially developing its version of (free) Mach, MK++, for use by its members' (commercial) OSes (see the white paper MK++: A High Performance, High Assurance Microkernel."
Mach4 is now developed by GNU as the continued basis for the HURD, its free microkernel-based OS.
The Flexmach was a project to implement objects above Mach in C++ (yuck) according to the related OMOS model and implementation over plain Unix (people at University of Utah have stopped maintaining Mach4, as their terminated Flexmach, and concentrate on their new Flux project and Fluke microkernel).
This page is linked from: Chorus HURD Lites Mac OS X MachTen Microkernel Microkernel Debate MkLinux Opal rtmk