The Xen Project hypervisor is an open-source type-1 or baremetal hypervisor,
which makes it possible to run many instances of an operating system or indeed
different operating systems in parallel on a single machine (or host). The Xen
Project hypervisor is the only type-1 hypervisor that is available as open
source. It is used as the basis for a number of different commercial and open
source applications, such as: server virtualization, Infrastructure as a Service
(IaaS), desktop virtualization, security applications, embedded and hardware
appliances

WWW: http://www.xenproject.org/