Thunderhead Explorer: MongoDB + Spring + Mobile Flex API for ArcGIS = Harmonie | Everything about Flash |

I've used MongoDB on a project for the City of Chicago with great success. I was impressed with the fact that we can store JSON documents in one giant collection, scale horizontally by just adding new nodes, the plethora of language APIs (Java,AS3) that can talk to it, run MapReduce tasks, and my favorite is that you can create a true spatial index on a document property.