Lincoln Mullen's digital map of the spread of U.S. slavery in the United States is getting a lot of attention via social media. It is based on this 1861 U.S. Coast Survey map.
Mullen provides some history and context for the 1861 map here.
Click here to play with his digitized version of the map. It allows the reader to trace the spread of slavery by decade in nearly every United States county. Fascinating stuff. My home county of Morris, New Jersey had 19 slaves as late as 1850. The county where I currently live--Cumberland, PA--had 24 slaves as late as 1840.