An early influential Google Maps mashup now gives users the chance to change mapping imagery. The GMaps Pedometer (our GMaps Pedometer mashup profile) has six different tile options, the most recent of which is OpenStreetMap, the free editable map of the world.
Almost every app that uses a mapping API needs latitude/longitude points. To get geocodes you need to convert an address to a point on the earth. That's where you'll need one of the 54 geocoding APIs in our directory. This post covers seven free geocoders and compares them on several aspects.
Mashup games are always fun. Those built upon FourSquare have an extra layer of enjoyment, because they're tied to the real world. In the case of City Warfare, players use various water weaponry to soak other FourSquare users.