A time machine
The What Was There website can be your personal time machine. Use it to see old photos (then) overlaid onto a current Google Map Street View (now).
On the site I found a 1919 photo of the West Side Market and clicked to layer it over a present-day street view.
The photo's "Fade" control lets you gradually time-travel back and forth between views.
As of October 2011 the site has 492 photos of New York, 46 of Cleveland, and only 7 of Paris, but that will change. The site's creators, Enlighten Ventures, encourage everyone to upload their own photos to "weave together a photographic history of the world."
An exciting idea, thoughtfully designed, and easy to use. I think it's a winner.