Foursquare just made the switch from using the Google Maps API, to joining the crowd-sourced, global OpenStreetMap movement. To do most of the heavy lifting, foursquare relied upon the still fairly new MapBox API.
It's no secret how big map mashups are on ProgrammableWeb. That means we've seen many and the really great ones stick out. In fact, two of these three mashups don't even use a map. But they all use location-sharing service Foursquare.