Berkeley Software Distribution of Unix (BSD)
Somewhere around 1975, the University of California at Berkeley purchased the AT&T code base and the rights to modify the code. Between 1975 and 1993, the software was under heavy development by Berkeley students and all of the old AT&T code was written out of the codebase. Why was this rewrite necessary? It seems that whomever buys the original AT&T codebase seems to turn into a rabid dog and starts suing everyone in sight for patent and copyright infringement. Thus, development continues to assure that BSD is completely original code.
There are three different flavors of BSD. OpenBSD, FreeBSD and NetBSD.
Open BSD holds as it's development goals the idea of open source code and secure software development. They claim that there hasn't been a root compromise of a BSD system software since the year 2000. They claim it's that secure. Personally, I think it hasn't been hacked because the script kiddies aren't that sophisticated yet and BSD isn't as widely deployed as Windows (which is trivial to hack).
Free BSD focuses on support for i386 systems (computers with Intel processors), but Intel systems are not the only hardware platforms supported.
This development branch of the BSD code aims at providing the smallest, fastest, most reliable networking platform available. Development of NetBSD is focused on scalability, embedded systems and parallel processing over the network.