Recently I gave a presented an overview of ModeShape 3, covering the basic features, deployment options, and best practices:
An open-source, federated content repository
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.