Drawing on the archives of Historic Map Works, the world's largest online collection of geo-coded historic maps, Historic Earth allows you to virtually travel back through time in many U.S. cities and states. Geo-coding allows the maps to closely overlay a modern map, so that the same locations can be compared in various time periods.

Launching with 32,000 high-resolution maps covering several cities and states (including NYC, Boston, Philadelphia, Washington DC, Maine, and Illinois) the collection will expand to 130,000 in the weeks after launch as more of our large archive is made available.

Historic Earth allows you to map the history of cities, times, buildings and landmarks. View historic maps showing property owners, see buildings constructed and replaced, and watch the landscape change over time.

More Info:
http://emergencestudios.com/historicearth (less info)