The Smithsonian Magazine website features a series of interactive historic map overlays showing the development of some of the major cities of the U.S. This seems to be an ongoing project involving maps from the David Rumsey Map Collection. So far, there are map comparisons for Chicago before and after the Great Fire, Denver, Los Angeles, Washington D.C., New York City and San Francisco. They are fun to look at, but because the new map is a satellite view of the modern city without street names or other markings, it seems to be of limited historical use in its present format. Perhaps, there will be some similar developments that will work better for researchers.
While you are thinking about maps, I suggest looking at the Geographical Searching with MapRank Search on the David Rumsey Map Collection website. It uses the same technology as the website OldMapsOnline.org.