Apache Sling
Code Location:

Ohloh Project Analysis
Apache Sling is a Web application framework based on REST principles that provides easy development of content-oriented applications. Sling uses a JCR repository, such as Apache Jackrabbit, as its data store. Sling graduated from the Apache Incubator on June 17th, 2009, and is now an Apache top-level project. All of Sling's code is under the Apache License 2.0 (quite obvious for an Apache project, but there was some confusion about this at some point).