The shortlist of graph databases I keep an eye on. Many other NoSQL engines can store graph-like data, of course.

Neo4j

Neo4j is a graph database boasting massive performance improvements versus relational databases. It is very agile and fast. At the moment it is used by many startups in applications such as  social platforms, fraud detection, recommendation engines etc. The data is stored in nodes that are connected by directed, typed relationships with properties of both (a property graph).

FlockDB

FlockDB is much simpler than other graph databases such as neo4j because it tries to solve fewer problems. It scales horizontally and is designed for on-line, low-latency, high throughput environments such as web-sites.

Twitter uses FlockDB to store social graphs (who follows whom, who blocks whom) and secondary indices. As of April 2010, the Twitter FlockDB cluster stores 13+ billion edges and sustains peak traffic of 20k writes/second and 100k reads/second.

GraphBuilder

Intel’s Graph Builder For Apache Hadoop reduces the effort to and speeds up time-to-insight, empowering data scientists to spend their valuable time using analytics to solve business challenges instead of formatting data. Use of popular and convenient Apache Pig scripting eases cluster compute programming. Graph Builder routines work alongside built-in Pig functions to simplify the tedious “data wrangling” around data import, parsing, cleaning, feature engineering, and graph construction. The output in widely supported data formats enables use with a broad range of graph database, visualization, and analytic tools.

InfoGrid

InfoGrid is a Web Graph Database with a many additional software components that make the development of REST-ful web applications on a graph foundation easy. InfoGrid is open source, and is being developed in Java as a set of projects.

InfiniteGraph

There is no other graph technology available today, offered by any other commercial vendor or open source project, that can match InfiniteGraph’s combined strengths of persisting and traversing complex relationships requiring multiple hops, across vast and distributed data stores.

AllegroGraph

AllegroGraph is a modern, high-performance, persistent graph database. AllegroGraph uses efficient memory utilization in combination with disk-based storage, enabling it to scale to billions of quads while maintaining superior performance. AllegroGraph supports SPARQL, RDFS++, and Prolog reasoning from numerous client applications.

HypergraphDB

HyperGraphDB is a general purpose, open-source data storage mechanism based on a powerful knowledge management formalism known as directed hypergraphs. While a persistent memory model designed mostly for knowledge management, AI and semantic web projects, it can also be used as an embedded object-oriented database for Java projects of all sizes. Or a graph database. Or a (non-SQL) relational database.

BrightstarDB

BrightstarDB runs on Windows, Linux, OSX, Windows Phone, Android and iOS, with a single consistent data file format across all platforms. The core libraries have a small footprint and install with zero configuration for embedded applications.

Sparksee

Sparksee (formerly known as DEX), makes space and performance compatible with a small footprint and a fast analysis of large networks. It is natively available for .Net, C++, Python and Java, and covers the whole spectrum of Operating Systems. Sparksee mobile is the first graph database available for iOS and Android.