I have added a new CLOUD project to the Xen.org community project page called AppScale from the University of California at Santa Barbara.
From Professor Chandra Krintz:
In the spirit of Eucalyptus (also from UCSB), we’ve implemented a cloud fabric (Platform-as-a-service) that emulates the Google App Engine cloud. Â You can find more information on it here:http://appscale.cs.ucsb.edu. Our system runs automatically over a Xen-enabled cluster (as well as Eucalyptus, EC2, and other virtualization layers).