The USA government is making an assortment of data available at data.gov. It can be filtered by category or agency.

data.gov http://www.data.gov/
tutorial http://www.data.gov/howtouse
search http://www.data.gov/catalog

Reminds me of this amazing video by the creator of the web on Data and the Next Web

http://www.ted.com/index.php/talks/tim_berners_lee_on_the_next_web.html

Apps for America 2 is a contest for developing applications using data from data.gov. The contest sponsors are Sunlight Labs, Google, O'Reilly Media, and TechWeb. All submissions are to be licensed under the MIT, New BSD or the GPL family of licenses.

contest: http://sunlightlabs.com/contests/appsforamerica2
apps: http://sunlightlabs.com/contests/appsforamerica2/apps
sunlightlabs google group: http://groups.google.com/group/sunlightlabs?pli=1