Neo4j: Intro to Graph Databases Webinar
Neo4j is a high-performance graph engine with all the features of a mature and robust database.
There are a number of reasons to consider graph databases important — In this webinar Emil Eifrém gives a great presentation on the benefits that graph databases can provide for complex data.
I've been researching Graph Theory over the past couple months and Neo4j appears to have hit a sweet spot for operating on graphs. Specifically, there are a class of problems that are exceedingly difficult to compute in a timely fashion on a relational database.