A graph database is an excellent place to stash data that involves relationships among its elements: Social networks, shipping routes, and family trees are a few examples that spring to mind. Neo Technology’s Neo4j is a graph database that’s simultaneously short on learning curve and long on scalability. Neo4j can be run either as an embedded Java library or in client/server fashion, which means it can tackle tasks small to large (a Neo4j database can house up to 34 billion nodes and an equal number of relationships).