FreeBSD

http://www.freebsd.org/gifs/freebsd_1.gif

What is FreeBSD?

FreeBSD is an advanced operating system for x86 compatible, DEC Alpha, IA-64, PC-98 and UltraSPARC« architectures. It is derived from BSD, the version of UNIX« developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.

http://www.freebsd.org/

Also see: BsD, OpenBSD , NetBSD