gCensus

Found this via Boingboing. This interesting lil' tool combines the best of Census data with the trendiest of Google tools. Zoom in on the map to see how many people live in a certain area. Of course if you want really specific info (also for free) you can always try AmericanFactFinder.

gCensus - Frequently Asked Questions: "The map tool located on the home page of this site is the result of a combination of technologies. The map images as well as the satellite imagery belong to Google and are pulled from maps.google.com. The data is extracted from the 2000 United States Census and can be downloaded from www.census.gov."