BSD
BSD (the Berkeley System Distribution) is a variant of UNIX which was designed at the University of California Berkeley. Since the original BSD, there has been at least four or five open source projects which have been grown out of the BSD tree.

FreeBSD is possibly the most famous. It runs Yahoo! and several others and has many of the top uptimes as measured by NetCraft. Its general focus is to be the best, most stable OS for the Intel platform.

OpenBSD is designed to be as secure as possible while maintaining support for multiple architectures, such as Sparc, Sparc64, Intel, IBM PowerPC, and many others.

NetBSD is the oldest BSD variant. Its focus can be summed up by the NetBSD motto: Of course it runs NetBSD! The NetBSD OS runs on about 50+ architectures and counting, from VAXes to Playstations to Dreamcasts and so many others.

Dragonfly BSD is the newest project, and is designed as a way to build on the current FreeBSD 4.9 source tree without the radical and revolutionary changes introduced into FreeBSD 5.0.