We have an open source database company in our portfolio. The company is called 10gen and the database is called MongoDB. This is a fairly techie subject and the pros and cons of data stores is not a topic I broach very often on this blog.
But MongoDB is worth talking about. As 10gen's marketing says, MongoDB contains "The best features of document databases, key/values stores, and RDBMSes in one".
MongoDB was built by some of the former engineers who scaled Doubleclick to be the largest ad server in the world. In the process they found out that traditional relational databases presented some hard scaling challenges and the experience left them asking if there was a better way. MongoDB is their "better way".
And now MongoDB is available as a hosted web service. So if you want to play around a bit with MongoDB and see if it's a good choice for you next web project, just go to MongoHQ and request a beta invite.
If you do that, I'd love to hear what you think. Either in the comments or via email if you prefer.