I recently came across the site Dear Photograph after seeing it on Yahoo News. If you missed it, check it out. The site is full of visitor-submitted photographs which consist of an old photograph superimposed over the same location in the present.
As I looked through the photographs on the site, it occurred to me that this would be a good idea for a family tree scrapbook. The next genealogy research trip you take, why not try to match up the location in old photographs and take a new picture.