Finding Population and other data for a defined area

I thought there would be a tool out there on the web where I could pick a location (address) and then pick a radius (10 miles, 20 miles - whatever) and get the population, demographics and maybe some other granular data about the area within the circle. Apparently there is nothing for free. Maybe that is a project / business waiting to happen - although, free is a tough climb. It looks like I can buy tools (CD/DVD) or buy a subscription to get down to "block" level. Not city block, but census block (used for elections etc).

The best I could do for free was a database at the US Census Bureau Factfinder . There is some good information here and it's probably close to accurate. ZipSkinny is another site that seems nifty. It pushes essentially the same data - presented by zip code. Zip code sorting could be more helpful or less helpful but it's another pile. ZipSkinny can deliver side by side comparisons - nice. The source is most likely the 2000 census. I can see a way to create the kind of data I'm looking for by making some assumptions. If I want to look at a relatively small region, say - Southeastern, NH it wouldn't be too tough. For example: if the 10 mile radius from a particular address swallowed up 15% of a neighboring town, we could say that 15% of the population was within the radius section. Yes indeed, we could be slicing off the heaviest concentration of a particular town or the lightest. This would fuzz our % of the town conclusion but it would still be a decent representation. If the radius clipped pieces of a few towns, the errors +(-) about concentration would tend to offset.

If you know of a site where this data is organized as I've described, let me know. That would be worth ten bucks to someone.