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 OSs (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, an effort at the University of Cambridge to create a super-duper distributed computing environment.