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.

"Most users at one time or another have dealt with hierarchical data in a SQL database and no doubt learned that the management of hierarchical data is not what a relational database is intended for. The tables of a relational database are not hierarchical (like XML), but are simply a flat list. Hierarchical data has a parent-child relationship that is not naturally represented in a relational database table. In this article we will examine two models for dealing with hierarchical data in MySQL"