_(Xen|http://www.cl.cam.ac.uk/Research/SRG/netos/xeno/xen/) is a virtualisation kernel to which Linux has been ported, NetBSD and XP are being ported, such that you can run as many of each of these *(OS)s (and others if they are ported or written) on a single box. I'm sure I've gotten some of the words wrong, but that's essentially what it appears to be.

It's part of _(Xenoservers|http://www.cl.cam.ac.uk/Research/SRG/netos/xeno/), an effort at the University of Cambridge to create a super-duper distributed computing environment.

