Neo4j is designed for storing and retrieving information on the relationships between items. The database can be used to store information about the relationships between users and pieces of content, as well. I imagine this would be a good foundation for building a recommendation engine for related content.
It includes a relationship visualizer, shown above, based on springy.
Alex Popescu of MyNoSQL writes "If my memory serves me right, Neo4j started as a library used internally for building content management systems." I wasn't able to verify this, but the Neo4j team did create a graph to demonstrate how the database could be used with a CMS.
Here's another screenshot of the backend: