OpenStack is an open cloud computing platform founded by Rackspace and backed by NASA, see openstack.org. It's to counter the proprietary cloud offerings of Amazon's AWS and Microsoft's Azure. Ok, so maybe this doesn't exactly go under the 'operating systems' menu item, but it's pretty close. From their meetup page:
This is the Atlanta OpenStack developer Meetup. We are a group of free software developers who are passionate about working together to help make the Openstack project a success!