USA.gov - links to government statistical sources
Data.gov - raw and geospatial datasets
FedStats - 100+ agencies providing data and trends on topics like: economics, population, crime, education, health care, aviation safety, energy use, farm production, etc.
MapStats - Statistical profiles of States, counties, cities, Congressional Districts, and Federal judicial districts.
geodata.gov: U.S. Maps and Data
A web-based portal for access to maps, data, and other geospatial services.
Statistical Abstract (online) - 2011 edition; older in paper Docs C 3.134: 1893-2011
Federal Reserve Economic Data - data and pulbications for US finance and trade
Bureau of Labor Statistics - extensive collection of statistics and reports broadly related to employment and economics
National Science Foundation - status of science & engineering including education, employment, funding, policies.
TranStats U.S. Bureau of Transportation Statistics
Aimed at providing "one stop shopping" for transportation data. Explore by mode, subject, or agency. Includes a Mapping Center for viewing and analyzing data geographically.
USGS Water Data for the Nation U.S. Geological Survey
Provides access to water-resources data collected at approximately 1.5 million sites in all 50 states, the District of Columbia, and Puerto Rico.