See http://freebsd.org/.

FreeBSD is the original BSD (Berkeley Software Distribution) ported to the PC. NetBSD and OpenBSD are early forks of FreeBSD. DragonFlyBSD forked more recently, to explore new approaches to SMP and clustering.

I’ve run it a lot as a server, and it makes a decent desktop OS as well, except that

FreeBSD has treated me very well for the past seven years.