Panoramio Photo Density Maps
Alexander Tchaikin has an interesting project on building density maps based on Panoramio photographs. The project's goal is to show on Google Maps the number of photos geo-tagged at particular locations. For example, the two screen shots above show the areas being photographed most in Moscow (unsurprisingly Red Square is a very popular spot with photographers).
Alexander explains how he created these photo density maps on his project website. He also has a number of kml files, so you can review the results of his analysis on Google Earth.