Nimbula is an Infrastructure as a Service (IaaS) software stack analogous in its target market and its business model to commercial software like vCloud. It sits alongside a number of open source software products like Eucalyptus, Cloud.com (Citrix) and OpenStac k(Rackspace et al.) as well as the Amazon Web Service, and other hosted services.
Nimbula is a relatively-late entrant to the market, and is in some sense a second-attempt at IaaS, since it was developed by a lot of the original team that built AWS. It is a venture-backed statrup with a significant funding base, and is quite early in its adoption curve. There is a free version (limited to 40 cores) but this is not an Open Source product.