This is a screenshot from Sightsmap, a new heat map made by Google. It merges Google Maps with Panoramio, Google’s photo hosting site with geographic markers. The map ranks the 15,000 most photographed locations on earth. The brighter the color, the more photographed it is.
You can find it here. As you click on locations, you’ll see Panoramio and Street View photos from that area as well as links to information.