Neo4j

Neo4j is a graph database. It is an embedded, disk-based, fully transactional Java persistence engine that stores data structured in graphs rather than in tables. A graph (mathematical lingo for a network) is a flexible data structure that allows a more agile and rapid style of development.Neo4j is released under a dual free software/commercial license model (which basically means that it’s open source under the AGPLv3 but if you’re interested in using it with closed source software, then you must buy a commercial license).

Neo4j has been in commercial development for 10 years and in production for over 7 years. 

You can think of Neo4j as a high-performance graph engine with all the features of a mature and robust database. The programmer works with an object-oriented, flexible network structure rather than with strict and static tables — yet enjoys all the benefits of a fully transactional, enterprise-strength database.