Data.Gov

URL: http://www.data.gov/

Pan-government web site for microdata (raw, uncalculated data) produced by the major statistical-producing agencies
Searchable by keyword and/or category, agency, and file type (xml, csv, kml/kmz, esri, and other)
As of June 2009, this is a fledgling web site with 204 data files, primarily from the Environmental Protection Agency and Internal Revenue Service; expansion expected