Neo4j is an open source graph database, storing data in the nodes and relationships of a graph.

An intuitive graph-oriented model for data representation. A disk-based, native storage manager completely optimized for storing graph structures for maximum performance and scalability. Neo4j can handle graphs of several billion nodes/relationships/properties on a single machine and can be sharded to scale out across multiple machines. A powerful traversal framework for high-speed traversals in the node space. Can be deployed as a full server or a very slim database with a small footprint (~750k jar). A simple and convenient object-oriented API.

Unlike twitter's FlockDB, neo4j can handle multiple levels of hierarchy. However where FlockDB is better is it's low latency, high throughput and inherent distributed nature.