A PhD candidate from Brandeis University came up with this interactive map showing the spread of slavery in the United States. I'm putting it here so I can book mark the page, but also because its a great tool for both adults and youth to use as we explore how slavery develops where it does.
The idea of doing this kind of digital history with my students fascinates me...I want to learn how to teach this to them.
BTW-Lincoln Mullen's blog is interesting apart from the map. You should check it out.