The last two years of my working life I been at the UPC , specifically with the DAMA-UPC research group: the data management experts of this university. One of the lines of research is graph databases, which is the main topic of this post.
In the same way that relational databases organize data in the form of tables, graph databases do this in the form of a graph, or a network. Nodes, edges, attributes and algorithms are the objects of interest in this field.
If we have a new project where a database is needed, the main reason that would drive us to graph databases is if the importance resides on relationships versus the raw data. Considering the most trendy applications, social networks, data mining, etc. they all meet this condition. A final feature of these databases, which is also shared with other NoSQL solutions, is the schema-less property; we've no obligation to define the complete schema. The most important graph databases nowadays are:
Neo4j is a graph database. It is an embedded, disk-based, fully transactional Java persistence engine that stores data structured in graphs rather than in tables. Neo4J provides you with API's for Ruby, Python, and Java also with support for various web technologies.
Dex: The high performance graph database.
Dex is the graph database developed at DAMA-UPC , a research group of the Technical University of Catalonia. If you need to store huge amounts of data and high performance queries are mandatory for your applications, DEX is your graph database.
HyperGraphDB is a general purpose, extensible, portable, distributed, embeddable, open-source data storage mechanism. It is a graph database designed specifically for artificial intelligence and semantic web projects, it can also be used as an embedded object-oriented database for projects of all sizes.
The Internet graph database
InfoGrid is the open source internet graph database with many additional components that make it the ideal tool for the development of REST-full Web applications.
Now you only need to try them to see what they could provide your projects.