Why XEN rocks
 

Xen, an opensource virtualization project, runs faster than:

1. VMWare
2. UML
3. Virtuozzo

Since it's opensource, it's free. Run any distribution you want under it, provide your users with the choice between Debian, RedHat, Ubuntu, Gentoo, etc. Server getting to full? Migrate the VM to another server while it's running. Use this with LVM and you have one badass server.

Why would you not want something this awesome :)