The US Census is a treasure trove of valuable information for many people. From salesmen trying to market to curious people trying to learn about new cultural trends, the Census is a great source of information. Accessing this data in a computer readable form is sometimes difficult, however. The USA Today Census API simplifies the process, at least for the 2010 census.
From USA Today's announcement:
Since its founding, USA TODAY has focused on covering the entire country, not just one region or group of people. We cover who the American people are and how they live no matter where they live. One way we do this is to pay a lot of attention to demographic data. Our reporters analyze it constantly and talk to many people about what it means.
That's why we're excited to share the USA TODAY Census API. It's a simple way to pull highlights from Census 2010 for tens of thousands of places. For these data points, we also have data available from Census 2000 to allow comparisons (though open access here is limited to 2010 data, please contact us at firstname.lastname@example.org if you are interested in full access). Data includes basic demographics, such as population totals, and ready-to-use percentages for subjects such as race and housing vacancy.
You can search info about Population, Race, and Housing for any area you choose, using intelligent keywords. You can get all of the data about a place, or just one of those, with different calls. Access to population history is available, but requires contacting them, and most likely paying an unspecified amount. Even with just the 2010 data, one could figure out a lot of useful things.