Below are some of the most relevant Federal agencies. (Here is a complete list.)
This is almost certainly the most comprehensive database of Congressional publications, some dating back as far as the late 1700's. Includes the complete texts of hearings and most other documents. An interesting sub-set of the database provides legislative histories. (Law Library subscription.)
Probably the single most useful published collection of statistical information on virtually all aspects of life in the United States. And it usually directs you to the originally-published sources of data for more in-depth coverage.
Earlier editions also are available online from the Census Bureau. Printed copies (1878-2011) are available in the library's Reference collection.
Contains references to tens of thousands of data sources from the U.S. Federal Government, U.S. state governments, international intergovernmental organizations, and private sector groups, such as trade and professional associations.