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.)
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.