MongoDB is a scalable, high-performance, open source NoSQL database. It fits a lot of typical web site needs. Well, we know a software that can be used to build spectacular websites, don't we? So it was somewhat a given that the two should meet. And they did: at DrupalCon San Francisco, three years ago already Doug Green and me have showed how Examiner.com was using MongoDB for serious performance benefits. However, with Drupal 7 it was pretty much a given that you needed MySQL besides MongoDB, single database operation was not viable.

It was already a given that Drupal 8 will be even more friendly to MongoDB when 10gen, the MongoDB company provided generous sponsorship to make sure you can use MongoDB alone cleanly and nicely. See in this presentation what challenges we faced with Drupal 7 and how they were solved in Drupal 8 and how MongoDB will work with Drupal 8.

Note: the slides are very preliminary.