The U.S. Census Bureau has just launched an interactive online map widget that lets you explore both local data as well as 10 decades of population growth at the country level. There are currently four states available for the local view (Louisiana, Mississippi, New Jersey, and Virginia), with more being added as their data become available. The display includes population totals as well as data on race and age.
The widge-based approach allows you to even embed the viewer within your own website, with updates occurring automatically as the data is released. A more detailed view of the same data is available via the American FactFinder site.