Neo4j is 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. For
many applications, Neo4j offers performance improvements on the order of
1000x or more compared to relational DBs.

Neo4j is an open source
project available in a GPLv3 Community edition, with Advanced and
Enterprise editions available under both the AGPLv3 as well as supported
by Neo Technology with a commercial license.