OpenNebula has just announced the release of the final version of the Hyper-V drivers for OpenNebula 3.2.  As far as is known, this constitutes the most feature-rich, scalable and efficient integration of an open-source cloud stack with Microsoft Hyper-V, being an excellent example of collaboration in cloud innovation and interoperability.  Now Microsoft Hyper-V clouds can be accessed by using standard and common cloud APIs like OGF OCCI or AWS EC2.

See http://blog.opennebula.org/?p=2661 for additional information.