Xen is a hypervisor based virtualization technology originating at the University of Cambridge, nowadays developed largely by the company [http://www.xensource.com/ XenSource]. Xen introduced the concept of paravirtualization, which allows for extremely high performance virtualization provided that the guest virtual machine runs a modified operating system kernel.
Xen also supports full virtualization (running unmodified operating systems) on processors with Intel VT or AMD-V technology.
Xen is available in many Linux distributions, however ["lhype"] and ["KVM"] appear to be making more progress at getting merged into the upstream kernel...
[http://jailtime.org/ Jailtime] a site with many different Xen guest images.