We have implemented it for internal purposes and more for learning.
Integrated with KVM and VMware Hypervisors and on meager resources, just to check whether it works or not. We started this approximately five years back and it worked flawlessly.
A YouTube video shows the details of our implementation.
This can be used as a private/hybrid cloud and there are many well-known organizations that are using OpenNebula. These include the European Space Agency (ESA), BBC, China Telecom, TransUnion, and Trivago.
OpenNebula can also be used for High-Performance Computing.