NetBSD 1.5ZB
Dahan writes: "I just saw that the development branch of NetBSD is now at version 1.5ZB. A change log is available for those interested. Note that although the title of the page says it's a list of changes from NetBSD 1.5 to 1.6, NetBSD 1.6 is not out yet--the page lists changes that will be in 1.6 whenever it's released. (And when will that be? "When it's ready," of course.) Standard caution about not running development kernels on mission-critical systems applies, although I've been running 1.5ZA on my DEC^H^H^HCompaq Alpha PC164 web/mail/DNS/whatever server for a few months now, and it's been great. And for those of you used to the Linux version numbering scheme and are wondering what all these letters mean, here's an explanation of NetBSD's version numbering."
NetBSD's got a very nice rc (startup) system; as opposed to the monolithic (Open|Free)BSD approach, NetBSD's is a highly modular dependancy based model; no more giving scripts esoteric names like "000.wibble" to try to get it executed before "001.wobble"; just add a dependency in wobble on wibble and the rc system will make sure wibble is executed first.
There's an interesting PDF paper on the design and implimentation, some conciderably more terse and less interesting official documentation and a Daemon News article, and for those uber geeks, the CVS repository where you can compare with the other BSD's.
You'll note FreeBSD -CURRENT is looking at adopting it, while Open sticks with the tried and tested BSD4.4-type setup