If you're ever thinking of moving beyond SQL and the relational database world, this carsonified.com article is well worth your time.

It covers most of the major players, including Hadoop, CouchDB and Sesame.

The comments are also worth a look because they cover a few areas left unexplored in this post, particularly XML databases. They didn't mention my personal hobbyhorse, eXist, but there's some others there I hadn't heard of such as M/DB:X.