Linux has a article on Xen, a virtualization system similar to user mode linux, which I did a report on a while back. Xen is used by Unixshell, a company that provides web hosting on virtual machines (no affiliation with them, just noting they use the same tech).
Here’s a link to some Xen multi-distros-running-all-at-once screenshot goodness.
Anyway, good article, Xen is something I want to check out in the near future. Link to Xen and Gentoo HOWTO.