FAQ: List of threadless operating systems and recommended operating systems are wrong
If you're running on Solaris, OpenBSD, NetBSD, or old FreeBSD, Tor is probably forking separate processes rather than using threads.
That's not really true: most (all) of those have kernel threads now, and I think that we use threads everywhere besides Solaris so long as it has pthreads at all.
Tor relays work best on Linux, FreeBSD 5.x+, OS X Tiger or later, and Windows Server 2003.
That list can't be right, can it? I think we'd recommend more recent FreeBSD, OSX, and Windows. And we should really distinguish server from client OS.