Citrix

XenSource, Inc was acquired by Citrix.

http://citrix.com/xenserver

http://www.linux-mag.com/id/7252

http://gigaom.com/2008/06/26/citrix-cant-stay-xen-as-microsoft-launches-hyper-v/

For alternatives see VirtualBox, Qemu, Solaris Containers, or User Mode Linux.