The complete story of the ☞ Scrabb.ly, a multiplayer online Scrabble, built during the 48 Node Knockout. The only part about MongoDB:
As soon as we came up with the idea, we knew exactly the right tool for this application. MongoDB geospacial indexing was a lifesaver here. We are able to store every tile thats placed on the board by its x,y coordinate and query a bounding box whenever someone changes their view. The resulting queries are ridiculously fast, even with a tile collection approaching over 90 thousand. Once we get too big for that, scaling is going to be dead simple given we can just use modular arithmetic to produce a shard key.
At least the second project that puts MongoDB geospatial support above all other features (nb if I remember correctly the first I hear was foursquare).
Original title and link for this post: Node.js and MongoDB for Multiplayer Online Scrabble (published on the NoSQL blog: myNoSQL)