FreeBSD is an advanced operating system for x86 compatible (including Pentium® and Athlon™), amd64 compatible (including Opteron™, Athlon 64, and EM64T), Alpha/AXP, IA-64, PC-98 and UltraSPARC® architectures. It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.

I've been using FreeBSD since 1999. I like it a lot. Right now I run 6.x and am tracking -STABLE on two machines, and I upgrade them every month or two. A third box runs 4.x and I leave it alone.