Eucalyptus - Open Source Cloud Platform

The most installed open source cloud computing system. Initialization requires only few command, but complex installation requirement & complicated codebase are barriers. Excellent hybrid cloud platform.
Eucalyptus – a software platform for the implementation of private cloud computing on computer clusters, exports a user-facing interface that is compatible with he Amazon EC2 and S3 services.  But the platform is modularized so that it can support a set of different interfaces simultaneously.