“Federated Cloud Computing – The OpenNebula Experience” is the title of our keynote at the GARR 2012 Workshop on Distributed Computing and Storage. Given the scope of the workshop and the activity of GARR in the operation of the national high-speed telecommunication network for Research, the talk mostly focused on private cloud computing to support Science and High Performance Computing environments, the different architectures to federate cloud infrastructures, the existing challenges for cloud interoperability, and our vision for the future of existing Grid infrastructures.