Xen:
Xen is a virtual machine monitor for x86 that supports execution of multiple guest operating systems with unprecedented levels of performance and resource isolation. Xen is Open Source software, released under the terms of the GNU General Public License. We have a fully functional port of Linux 2.4 running over Xen, and regularly use it for running demanding applications like MySQL, Apache and PostgreSQL. Any Linux distribution should run unmodified over the ported OS.
This is excellent news for those of us who would like to tinker with server/OS virtualization but don’t have the dough to throw down for VMWare Server or other high dollar software packages.
Aparently you can run pretty much any Linux distro on top of Xen, and they are also very close to a port of WindowXP that would work on top of Xen. You can download a demo CD and check it out.
This is truly cool stuff.