illumos is worth a mention then; it's a fork of OpenSolaris. Much of the top Solaris talent left after Oracle acquired Sun and moved to companies that contribute to illumos.
OpenIndiana is a ready-to-run distro moving to illumos, although it's still raw.
I normally wouldn't care about Solaris, but after having spent some time with its tools, I wish those ideas would spread more. rafekett is right, it is a killer server OS if you're willing to step beyond the familiarity of Linux, and hopefully illumos becomes more popular now that the OS isn't laboring under a single company.