Nova is a cloud computing fabric controller (the main part of an IaaS system) built to match the popular AWS EC2 and S3 APIs. It is written in Python, using the Tornado and Twisted frameworks, and relies on the standard AMQP messaging protocol, and the Redis KVS.
Nova is intended to be easy to extend, and adapt. For example, it currently uses an LDAP server for users and groups, but also includes a fake LDAP server, that stores data in Redis. It has extensive test coverage, and uses the Sphinx toolkit (the same as Python itself) for code and user documentation.