The Past Superimposed on the Present.

Take a look at What Was There: This website adds photos to places, tagged by location and year.  If there is a street view in Google maps, the photograph is  lined up from the same point of view to provide a glimpse of the same place at a different time. You can see how the streetscape has changed, as well as the architecture, car design, and fashions.

The site even allows you to fade the photograph in and out.  Cities in the United States have more photographs than other parts of the world, but I am sure that will change as more people travel the site. You can upload photographs yourself, although I have not tried that yet. This could be useful for preservationists, architects, and urban planners.