My Idea: You are walking on 5th Avenue in New York and wondering how this street looked 20 or 100 years ago. Of course you can look up photos in relevant books and obtain an impression of how life was at a certain point in time. But wouldn’t it be great to be able to transform this impression and get a detailed picture; to be able to literally see more and truly visualize your impression? In addition to the one or two photos from old magazines or in history books, there must be other historical documentation, which can assist in creating the picture. 
Back to 5th Avenue. A place which has existed for more almost 100 years in its current form; which has been visited by a vast number of people at least once. As tourists like to do, they photograph the places they visit to show family/friends, where they have been
The sum of photos of the street would form a breathtaking historical view of the place:millions of pictures showingall small/large changes throughout decades

cool

I sort of did that with this photosynth, incorparating historic images. http://photosynth.net/view.aspx?cid=14d91e89-e919-4681-9282-4f813cd69b69