Data.gov

Data.gov provides access to high value, machine readable datasets generated by the U. S. Executive Branch of the Federal Government. Information in searchable data catalogs can be accessed in three ways: through the raw data catalog, the tool catalog, and the geodata catalog.

This is a great site for info geeks. Search by keyword, single or multiple categories, and/or single or multiple agencies. There are datasets for a wide range of subjects including agriculture, business, climate, ecology, administrative and political boundaries, imagery and base maps, military, locations and geodetic networks, oceans and estuaries, transportation networks, utilities and communications, and much, much more.