What We Pay For is a simple website that lets you enter your income and filing status and find out how your tax dollars are being spent. It breaks down the amount you're likely to pay in taxes by various spending categories, such as Social Security, national defense and Medicare.
The contest opened today and will close on March 27, 2011. First prize, chosen by a jury, will receive $5,000. Not a huge sum, especially after you subtract taxes, but it's more than I'm getting back this year. And really, many worthwhile projects could come out of using the What We Pay For API, regardless of the contest.
Here's a screenshot from an interactive visualization that Google built:
You can find more examples here.