Virtualization and Linux--A Primer (Part 2) - page 2
User-Mode Linux runs multiple guest Linuxes on a host Linux system. The guest Linuxes run as applications in userspace. These days most distributions should have kernel support for UML built-in, or have packages that contain UML-enabled kernels. You can run any Linux distribution as a guest.
UML has all kinds of uses: shared Web hosting, cross-distribution development, kernel testing and debugging, network testing, honeypots, and running many services on a single machine in safe discrete environments. It's free of cost and free software, licensed under the GPL. It is plenty stout and stable, and while I've not seen any slick management consoles for it, it's pretty easy to use.