Dataplace is huge clearinghouse of demographics & mapping information gleaned from the US Tiger Census. For example, you can get the census of any region/city, including a slick use of sparklines, as well as a map of the location demographics spread.

An incredible feature is the ability to build up your own thematic maps by overlaying various layers of information at your own choosing. For example, you can build a map, of say, overlay the “Percentage Housing Units that are Overcrowded” with the “Aggregated Family Income”.

Like any good project nowadays, they have a Dataplace developer’s blog.
Dataplace forums.