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FreeBSD is an advanced operating system for a variety of architectures. It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley; and is developed and maintained by a large team of individuals. It is a complete Operating System, including the kernel, drivers, and userland utilities such as shells.

FreeBSD includes many modern OS features including advanced IP stacks and networking, Virtualization, hardened security, and high availability techniques.
For more information about FreeBSD, see their website (FreeBSD.org).

Questions are welcome here on Server Fault, the Unix and Linux Stack Exchange or the FreeBSD Forums.

Additionally Wikipedia has en entry on FreeBSD and the original BSD OS.