I’m very interested in Eucalyptus and OpenNebula, since they are both  open source, they have many features, they both support Amazon EC2 and  Amazon APIs and they seem to be able to work with different hypervisors, such as KVM, Xen, VMWare ESXi, etc. Eucalyptus seem to be  very supported by Canonical, because it’s a part of Ubuntu Server, so I bet it will grow more and more, and we have a good experience with Linux and in particular Ubuntu. On the other hand, many people told me that right now OpenNebula is better. So the question is: which one between Eucalyptus and OpenNebula? The other three choices? vSphere is probably free (if we use ESXi and not ESX) but it’s not open… abiCloud seems to be similar to Eucalyptus and OpenNebula… about Joyent, many people say very good things, but I want to know more, and then, is it open source? 

After having built the private cloud, we probably want to also use an external service and create an hybrid cloud. If for private cloud we chose OpenNebula,Eucalyptus, abiCloud or Nimbus I guess the best thing is to use Amazon EC2. Is it also possible to use VMWare solution? And if we chose abiCloud or Joyent, what can we use to create an hybrid cloud? About Microsoft Azure and Google App Engine, it seems to me they have many limitations, so I’m not considering them. What do you think about them?