A Property Graph Model Interface (no longer active - see Apache TinkerPop)
JanusGraph: an open-source, distributed graph database
OrientDB is the most versatile DBMS supporting Graph, Document, Reactive, Full-Text and Geospatial models in one Multi-Model product. OrientDB can run distributed (Multi-Master), supports SQL, ACID Transactions, Full-Text indexing and Reactive Queries.