Splendor [schwinki.de] is a great example of how much can be learnt by combining two simple data sets in a creative way. In this project, the positions of photos of landmarks (such as monuments, famous buildings etc.) are connected to the geographical position of the landmark itself. As a result, each landmark can be characterized with respect to its "best viewed angle", the distance at which it is typically photographed, etc.
As the basis of the data, the titles of geolocated photos from Panoramio were matched with a search for hotspots on Qype. Simple and effective. Then, individual maps were created for a number of cities. In particular, make sure to check out the zoomable example of Potsdam.