ModeShape is an open source implementation of the JCR API from Red Hat.
ModeShape is a lightweight, fast, pluggable, open-source JCR repository that federates and unifies content from multiple systems, including files systems, databases, data grids, other repositories, etc. Use the JCR API to access the information you already have, or use it like a conventional JCR system (just with more ways to persist your content). ModeShape used to be 'JBoss DNA'. It's the same project, same community, same license, and same software.
Home Page: http://www.jboss.org/modeshape
Downloads: http://www.jboss.org/modeshape/downloads