Linux and Open Source technology
OpenStack started out as an open source project by RackSpace and NASA in 2010. It was designed to enable any organization to run cloud services on standard hardware. IT industry companies like AMD, Intel, Red Hat, Cisco, Dell, HP, IBM, VMware, Yahoo and a bunch of others have joined the project. OpenStack is a suite of interrelated software packages that control processing, storage and networking resources throughout a datacenter. Administrators can manage these resources through a web accessible dashboard, and users can provision resources themselves. All of the code for OpenStack is freely available under the Apache 2.0 license. Anyone can run it, build on it, or submit changes back to the project.
Platform-as-a-Service is getting a lot of press lately, and with good reason. PaaS is an efficient way to develop applications, as you can start up a new development machine in a matter of seconds and take it down when you're finished with it. Or cater to specific needs easily, such as combinations of framework versions and databases.
Red Hat's Openshift is such a Platform-as-a-Service, ready for production after two years of beta testing.