Mach is a free *(microkernel) upon which many experimental *(OS)es are built.
There exists "ports" of classical *(Unix) to Mach,
_(BSD) (_(Lites)) and _(Linux) (_(MkLinux)),
consistently slower than their "monolithic" counterparts.

The original
<A HREF="http://www.cs.cmu.edu/afs/cs.cmu.edu/project/mach/public/www/mach.html"
  >CMU Mach</A>
project is now ended with Mach3
(see
<A HREF="http://www.cs.cmu.edu/afs/cs.cmu.edu/project/mach/public/www//doc/publications.html"
  >Published and Unpublished Mach Papers</A>).
Some pretty interesting ones are available
<A HREF="http://passaic.cs.miami.edu/os/Mach/"
  >here</A>.
Two different groups are working
on two divergent further versions of it:

<A HREF="http://www.osf.org/"
  >The Open Group</A> (formerly known as the Open Software Foundation (OSF)) is officially developing its version of (free) Mach,
<A HREF="http://www.opengroup.org/ar/technologies/mk-dbleplus/index.htm"
  >MK++</A>, for use by its members' (commercial) _(OS)es (see the white paper <A HREF="http://www.opengroup.org/ar/technologies/mk-dbleplus/white_paper.htm"
  >MK++: A High Performance, High Assurance Microkernel.</A>"

<A HREF="http://www.cs.utah.edu:80/projects/flexmach/mach4/html/Mach4-proj.html"
  >Mach4</A> is now developed by _(GNU) as the continued basis
for the _(HURD), its free _(microkernel)-based _(OS).

The <!--A HREF="http://www.cs.utah.edu/projects/flexmach/"
  -->Flexmach</A> was a project to implement
_(object)s above Mach in _(C++) (yuck) according to the related
<!--A HREF="http://www.cs.utah.edu/~mecklen/omos/iwooos.html"
  -->OMOS</A> model and implementation over plain _(Unix) (people at University of Utah have stopped maintaining
<A HREF="http://www.cs.utah.edu:80/projects/flexmach/mach4/html/Mach4-proj.html"
  >Mach4</A>, as their terminated Flexmach, and concentrate on their new
_(Flux) project and Fluke _(microkernel)).
