FreeBSD http://www.freebsd.org/

FreeBSD is an advanced operating system for Intel ia32 compatible, DEC Alpha, and PC-98 architectures. It is derived from BSD UNIX, the version of UNIX developed at the University of California, Berkeley. The 4.x series is the Production Release, while the 5.x release is the potentially less stable Technology Release.

Note: FreeBSD is not Linux. It is based on BSD UNIX. Please don't send us angry emails telling us this.