The personal finance site Mint aggregates spending data from four million users. At the individual level, Mint is useful in that it brings all of your finances into one place. Zoom out and aggregate, and you have spending for a city or a state. This is what Mint Data does.
Look up your city, and see how people in you area are spending their money. Where do people eat? Where do people shop? How much do people spend at different stores?
The above is overall spending trends for those in San Francisco. Notice the spending spike during the holiday season.
There are still some bugs, such as popularity rankings of zero and some monthly expense values at the state level that seem way too high. It should be easy to see the possibilities though. As we've seen from OkCupid, there's a lot of interesting things you can find when you have data for millions of users.
[Mint Data | Thanks, Jason]