Neo4j

Neo4j is an open-source graph database, implemented in Java. The developers describe Neo4j as "embedded, disk-based, fully transactional Java persistence engine that stores data structured in graphs rather than in tables". Neo4j version 1.0 was released in February, 2010.[3] The community edition of the database is licensed under the free GNU General Public License (GPL) v3. The additional modules such as online backup and high availability, is licensed under the free Affero General Public License (AGPL) v3. The database, with the additional modules, is also available under a commercial license, in a dual license model