HyperGraphDB is a extensible open-source graph-based data storage engine.
Graph
http://www.hypergraphdb.org/
https://github.com/hypergraphdb
http://www.hypergraphdb.org/?project=hypergraphdb&page=LearnHyperGraphDB
Kobrix Software
US
2007
Open Source
Java
Berkeley DB
Linux, OS X, Windows
Apache v2