"This cool new historic mapping app from the folks at esri and the U.S. Geological Survey is worth exploring. What it does is take 100 years of USGS maps and lets you overlay them for just about any location in the nation. That allows users to see how a city – say Harrisburg – developed between 1895 and today. The library behind the project includes more than 178,000 maps dating from 1884 to 2006."
Via Seth Dixon