If you are Digg or LinkedIn you can build your own speedy graph database to represent your complex social network relationships. For those of more modest means Neo4j, a graph database, is a good alternative.
After explaining the core concepts of graph databases and why they provide a good way to store data the article goes on to describing the features of Neo4j and the status of the project. The comments section of the article contains some good additions to the article by Emil Eifrem, CEO of Neo Technology (the commercial backer of Neo4j) with pointers on how to build real applications using graph databases. The comments also feature the beginnings of an interesting discussion on the role graph databases play in the future of the web."