[kuwze]

I really like NetBSD. It supports Xen[0], it has the awesome multithreaded NPF[1], the CHFS filesystem tailored for SSDs[2], and support for rump kernels[3].

[0]: https://wiki.netbsd.org/ports/xen/

[1]: https://en.wikipedia.org/wiki/NPF_(firewall)

[2]: https://en.wikipedia.org/wiki/CHFS

[3]: http://rumpkernel.org/