A free Unix based on the BSD(I) code. In my experience with it, it is a lot more straight forward about things, and faster than Linux. It currently supports x86 only, although other ports are planned. Full source is provided under either the Berkley license of the Gnu GPL (depending on the program in question).
see: http://www.freebsd.org for a start
CNET included FreeBSD in it's recent article on alternative OSes: http://www.cnet.com/Content/Reviews/Compare/AltOS/ss02.html
Relavance to things JOS: Good platform to develop on :) Kernel configuration is handled in two interesting manners Ports idea is brilliant.. FreeBSDLicense