MongoDB is an open source database, one of several called NoSQL systems (read about that in this post by Paul Ramsey). It’s interesting in geo since it has a geo type and has a new release, 1.6. And, it’s also interesting because it powers Foursquare.
Why do I bring this up? Mostly because I suspect a large proportion of readers have yet to hear about this new class of databases and their role in the social networking space. MongoDB has friends named CouchDB and Cassandra. I can’t say I fully understand how these may play out in our work, but I think readers should know they are out there powering some big implementations.