CloudStack

http://cloudstack.apache.org/
KVM, Xen & VMware support

oVirt

uses libvirt

Cloudmin

http://www.webmin.com/cloudmin.html

built on top of Webmin

Cloudmin is designed for use by VPS hosting companies that sell virtual systems to their customers, but is also suited for anyone who wants to get into virtualization for application management, testing, controlling a cluster of Virtualmin hosts, or just to learn about cloud computing.

ConVirt

Xen & KVM; formerly known as xenman     GPL v2