1. AllegroGraph. http://franz.com/agraph/allegrograph/.
2. Apache TinkerPop. http://tinkerpop.incubator.apache.org.
3. Cypher - the Neo4j query Language. http://www.neo4j.org/learn/cypher.
4. InfiniteGraph. http://www.objectivity.com/infinitegraph.
5. Neo4j graph database. http://www.neo4j.org/.