The CSIS (Center for Spatial Information Systems) at The University of Tokyo has a research program on the modeling of people flows in urban areas by using GPS devices. Here, the map is really a video you can see on youtube on the flow of persons in central Tokyo during a day in 2008.

When you use other sources, as geotags for images uploaded to flickr and picasa, you get interesting maps too, as the one by Eric Fisher, part of a world atlas which somehow describes tourism interest in some areas. Two interesting maps, provided you remember what is said in their description, as not everyone moving in the city carries a gps, and many photos manage not to be posted on flickr or picasa…

A map of Flickr geotags in Madrid, by Eric Fisher, part of an Atlas of cities from all over the world