The official site of the U.S. Government Printing Office. Browse, retrieve by citation, or run a search for what you need. Retrieve documents in text or PDF, or download bulk data with metadata. Some newer documents are authenticated.
Official verions of federal regulations can be found at FDsys, but for a more convenient format try:
Legislative information from the Library of Congress includes bills, resolutions, activity in Congress, Congressional Record, schedules, calendars, committee information, presidential nominations, and treaties. More up-to-date than GPO Access for Congressional actions.
The Office of the Law Revision Counsel prepares and publishes the United States Code, which is a consolidation and codification by subject matter of the general and permanent laws of the United States.
Court websites often contain some court decisions as well as valuable information about the court.