Eucalyptus is an open-source cloud operating system that allows to deploy a private IaaS cloud with the industry standard Amazon API on basically any Linux x86 hardware.
At our university, we have built one such cloud from workstations in several computer classrooms. It serves us for experiments with the cloud environment as well as to leverage the computing power of the classroom machines for scientific computations of some other research groups.
Tomáš Vondra 