Parallel Kingdom is a new (and far from being released) GPS game based on Android platform. Since a long time locaion based games have been explored, but up to now none really succed to capture enough attraction, mostly because they required poeple to move to play the game which requires a lot of commitment to play the game in a cold winter day!
But this one, Parallel World, seems to contains some interesting concept. You can build virtual elements on top of the real world, and then influence other players. The exact mechanism of the game is not yet explained, but I like the concept, presented by this video.