The U.S. Census has just started to release the 2010 census data at the county level. Until now, only state level data was available. Data for 21 states is currently available with the rest to be released over next two months as it becomes available.
In addition to providing the data in CSV format, the U.S. Census has created several nifty interactive maps that can be viewed at their site or embedded in your site. (Embedding requires support for iframe tags.) The map widget includes three maps, population change, population density, and apportionment for each state for every decade for the past 100 years. It’s pretty nifty and an excellent example of how dense data can be made more approachable.
Image from U.S. Census