Oracle GNU/Linux 5.2 and Oracle Xen VM Server 2.1.2 ISOs are now available for download from:
http://www.oracle.com/virtualization
and you can get some Oracle Xen VM templates from
The Google App Engine JumpBox gives you a drop in place system for doing Google App Engine development that’s contained in an isolated environment within a virtual machine.
It runs exactly the same on GNU/Linux, Mac and other OS’s and gives you a development environment that lets you take advantage of great virtualization features like snapshots. It will run from a single download on Parallels Desktop, VMWare, Virtual Iron, Xen Open Source in fully virtualized mode and others.
according to http://www.novell.com/connectionmagazine the latest version (10.3) of openSUSE now includes VirtualBox & Xen virtualisation platforms
There is an OpenVZ live-CD VZ VM server with VZ VM guests (based on CentOS or KNOPPIX) that you can download from http://wiki.openvz.org/Download_live_CD
And there is a quick start guide to read at http://wiki.openvz.org/Getting_started_with_OpenVZ_live_CD