I see a lot of OpenBSD, and few NetBSD and FreeBSD and DragonflyBSD... The latter three are very cool too!

NetBSD has its own in-kernel virtual machine, but unl/ike OpenBSD's vmm(4), it uses qemu as a front-end (it is a qemu "accelerator"). It has ability to test kernel code in userland (rump kernel), great for drivers debugging. Instead of using Perl (OpenBSD) or Lua (FreeBSD) or anything, it uses sed, awk and make to get around. And its package manager is in C. Not to forget that nowadays, Minix is a kernel for NetBSD userland and libraries and network stack.

DragonflyBSD can run an entire kernel on userspace (no less!) and its upgrade model is "git pull; make"!
And of course, HammerFS...

FreeBSD has support for a LOT of things (including Linux binaries!) and has its own bhyve hypervisor in addition to jails. It also uses the same style of concise /etc/rc.d/ service scripts as OpenBSD.


Ahem... I use OpenBSD. *blushes*