This We Know is a new site with a grand goal: to synthesize all public government data into a single semantic web application. They've got a way to go, but the site does currently let you look up which areas have the highest unemployment, for example, or the most nomadic population. But the most compelling stuff on the site is probably the information about pollutants. You can type in your zip code and get a list of factories in your area and all the dangerous junk they emit, by weight.
http://www.thisweknow.org/
No pollutants (per the website...some geologists in the area would differ) in my area, population 296 (Census 2000, probably grown by a few).
