Is there an open source alternative to Vmware vSphere Server? Like a Ubuntu or similar Distro with virtualization hosting built in?

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http://askubuntu.com – I am looking at turning my old Dell 2650 and 2950 servers into Vmware vSphere Servers, however vSphere is very very expensive. Is there an open source alternative that provides the same performance as vSphere in a Ubuntu or Debian distro? I dont actually mean "Virtual box" or "Vmware player" but rather a server hypervisor that is easy to set up and host virtual machines on my network? (HowTos)