Eucalyptus

Elastic Utility Computing Architecture for Linking Your Programs To Useful Systems – is an open-source software infrastructure for implementing “cloud computing” on clusters. The current interface to EUCALYPTUS is compatible with Amazon’s EC2 interface, but the infrastructure is designed to support multiple client-side interfaces. EUCALYPTUS is implemented using commonly available Linux tools and basic Web-service technologies making it easy to install and maintain.

via Eucalyptus.