OrientDB is an open source NoSQL database management system written in Java. Even if it is a document-based database, the relationships are managed as in graph databases with direct connections among records. It supports schema-less, schema-full and schema-mixed modes. It has a strong security profiling system based on user and roles and support the SQL between the query languages. Thank to the SQL layer OrientDB is straightforward to use it for people skilled in relational world.
OrientDB uses a new indexing algorithm called MVRB-Tree, derived from the Red-Black Tree and from the B+Tree with benefits of both: fast insertion and ultra fast lookup.