GeoGuesser is my favorite kind of mash up. It shows you a random street corner somewhere in the world using Google Street View images. Your goal is to figure out where you are, using only the cues in the Street View images. Then place your guess on a map. The closer you are to the actual location, the higher you score.
The most difficult guesses for me have been long, desolate stretches of country roads, like the one pictured above. Give it a try, then let me know what your best tips for sussing out the right spots in the comments below. (via Kottke)