BSD
An advanced BSD UNIX operating system for “PC-compatible” computers, developed and maintained by a team of volunteers. There are several active BSD efforts. bsd.org
An umbrella site for all versions of BSD, both free and commercial.
FreeBSD
The home site for FreeBSD.
NetBSD
The home site for NetBSD.
OpenBSD
A free BSD with emphasis on portability, standardization, correctness, security, and cryptography.