The two major open source PaaS (platform as a service) offerings are Red Hat's OpenShift and Pivotal's Cloud Foundry. Like Cloud Foundry, OpenShift was designed to provide rapid self-service deployment of common languages, databases, Web frameworks, and applications. One of OpenShift's current differentiators is that continuous integration (using Jenkins) is a standard part of its workflow.
OpenShift is useful for both developers and operators, as well as for streamlining the process from development through testing and production deployment. Like Cloud Foundry, it can run in a public or private cloud, or on-premise.
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OpenShift comes in three versions: Online (no version number, but monthly updates), Enterprise (version 2.1), and Origin (version 3.0), all produced from the same upstream code. The Origin version is the bleeding edge, community supported, free open source edition. It receives daily updates and runs on your hardware using Fedora as the underlying operating system. You can download a preconfigured virtual machine, configure it yourself with Puppet or Vagrant, or even build it yourself from source code. Origin is not really intended for production environments, but can provide a good, fast, free development environment that runs on a laptop or desktop.