Yesterday I installed Free BSD 5.4 into a QEMU virtual machine on my Debian Etch system at home. It was an interesting experience, much quicker than Ubuntu which was the last thing I installed with QEMU.
It boots quite happily, even x.org works perfectly. I'm not sure what to do with it at the moment. BSD feels like Linux and tastes like Linux, but obviously it's not Linux, lots of things are slightly different. Stills it's a lot better than anything from Microsoft.