AnalyGIS and SRC, both of whom work on various tools for studying markets and communities, have teamed up to build a demographic study tool combining Google Maps (surprise) and 2000 US Census data. Click on a spot in the US, then select either basic census information (ethnic distribution, sex parity, and income averages) or housing information (owners vs. renters, housing value, age of units) within one, three and five miles of your target click. You can also enter an address directly.
They describe this as primarily a proof-of-concept exercise, so there's no telling when it will disappear. Still, for those of us who want a better way to access demographic information quickly and visually, this works pretty well. Since it's based on Google Map's public APIs and open access census data, it should also be relatively simple to rebuild should this one go away.
(Thanks, Joe Willemssen!)