neo4j - Meet Neo4j: The graph database platform powering today's mission-critical enterprise applications, including artificial intelligence, fraud detection and recommendations.
ArangoDB - A distributed open-source database with a flexible data model for documents, graphs, and key-values.
Apache TinkerPop - Apache TinkerPop is a graph computing framework for both graph databases (OLTP) and graph analytic systems (OLAP).
Cayley - Open-source graph database.
JanusGraph - JanusGraph is a scalable graph database optimized for storing and querying graphs.
Titan - Built like a hedge fund.