There are two major products that come out of Berkeley: LSD and UNIX. We don't believe this to be a coincidence.
-- Jeremy S. Anderson

Because AT&T stopped providing the SourceCode to UNIX with the advent of UNIX SystemV, the UCB continued to develop older versions of UNIX and eventually released their variant as Free under the BSDLicense.

AddToMe: the following should be a timeline -- http://www.applelust.com/alust/terminal/archives/terminal041202.shtml is a good reference to take from

4.3BSD_Net/2

The first almost clean release. Was released with 6 missing files so it wouldn't need a AT&T UNIX licence. (wich didn't help much anyway).

386BSD

Bill Jolitz free BSD wich was the fondation for NetBSD and FreeBSD.

BSD/386

BSDis first commercial release over wich they were sued together with University of Carlifornia by UNIX System Laboratories.

4.4BSD-Lite

The blessed release, everything based on this has a get out of jail free card thanks to Novell. This release is what came out from the USL suit. It has five major Forks:

FreeBSD

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.

NetBSD

NetBSD is a free, secure, and highly portable UNIX-like operating system available for many platforms, from 64-bit AlphaServers and desktop systems to handheld and embedded devices, and even PlayStation2.

OpenBSD

The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system. Our efforts emphasize portability, standardization, correctness, proactive security and integrated cryptography.

BSD/OS

The licensing issue clean commercial BSDi.

Darwin

AddToMe

Part of CategoryOperatingSystem