I heard that Mach + HURD can be rather resource intensive, but...
Mach. Mach is a free message-passing kernel being developed at CMU.
The Hurd servers, working with the GNU C Library, will provide Unix-like
functionality. Together with Mach they are the last major components
necessary for a complete GNU system. Currently there are free ports of
the Mach kernel to the 386 PC and the DEC PMAX workstation. (The PMAX
is one kind of MIPS-based DECstation.) Other free ports of Mach are in
progress. Contact CMU for more information if you want to help with one
of those or start your own. Porting the GNU Hurd and GNU C Library is
easy (easier than porting GNU Emacs, certainly easier than porting GCC)
once a Mach port to a particular kind of hardware exists.
Just though we should keep it in mind. (I think mach would handle MIPS cpu+
mips video real well 8-) )
I think BSD 4.4 still has some AT&T but I think a version runs on
the decstations (again mips).
Linux of course is 100% free but probably the least portable (it pains
me to say).
Bill 1st> Broadley@neurocog.lrdc.pitt.edu
Broadley@schneider3.lrdc.pitt.edu <2nd 3rd> Broadleyemail@example.com
Linux is great. Bike to live, live to bike. PGP-ok