U.S. Census Bureau
The U.S. Census Bureau offers a wealth of potentially useful resources. Here are some of the more noteworthy ones:
American FactFinder is the primary tool to find population, housing, economic, and geographic information/statistics.
County and City Data Book is the most comprehensive source of information about the individual counties and cities in the U.S.
Statistical Abstract of the United States is a comprehensive summary of statistics on the social, political, and economic organization of the U.S. (Statistical Abstracts before 2013 can be found here.)
U.S. International Trade Data includes U.S. export and import statistics, balance of trade by partner country, and state data.