FreeBSD is looking very attractive to me all of a sudden.

[edited to add Tahoe-LAFS]

[edited to add netmap]

Of course, it has had ZFS (http://wiki.freebsd.org/ZFS) and Dtrace (http://wiki.freebsd.org/DTrace) for a long time now.

More recently:
* Capsicum: “This provides a genuine, believably workable, migration path from existing systems to a brave new capability world. We can move one application (or even one library) at a time, without breaking anything.” - +Ben Laurie: http://www.links.org/?p=905 http://www.links.org/?p=973
* Twisted just landed (http://twistedmatrix.com/trac/ticket/1918) support for kqueue (https://en.wikipedia.org/wiki/Kqueue).
* netmap: http://queue.acm.org/detail.cfm?id=2103536
* It looks like they may soon reach full support for valgrind on FreeBSD: http://bugs.kde.org/show_bug.cgi?id=208531 (The current blocker seems to be that the valgrind devs need a clean green test suite so that they can notice when they accidentally break something. Hopefully the FreeBSD devs who are porting valgrind to FreeBSD will achieve that.)

So FreeBSD 9.0 just shipped (http://www.freebsd.org/releases/9.0R/announce.html), and look at some of the headline features:
* Kernel support for Capsicum Capability Mode, an experimental set of features forsandboxing support
* User-level DTrace

Frankly, not to attract flames, but I feel vaguely icky about depending on a permissively-licensed operating system. People who extend it and redistribute their derived work can withhold the source code of their extensions! Ick. But I guess I can get over that and give it a try.

UPDATE: bytestore@yandex.ru has voluteered to maintain the port of Tahoe-LAFS in FreeBSD. ☺ https://tahoe-lafs.org/pipermail/tahoe-dev/2012-February/007050.html