Those of you who have ancestors who lived in NYC, especially Manhattan, will love this. The New York Times website offers a map of historic 1811 Manhattan (created by John Randel)with an interactive overlay of modern Manhattan. That means you can switch back and forth between the two views to see the modern equivalent of historic streets and locations.
Also available in the same window is an 1830 mop of the farms in Manhattan, a map of street openings since 1642, and an overlay that indicates the growth of Manhattan based on historic census data.
You can play with the map at http://goo.gl/jdUAG
Thanks to Dick Eastman for revealing this site.