Sites that cover all of the three branches of government (Congress, Executive, and Judiciary), or that have wide applicability:
Because the US Congress generates a very large amount of information and publications, see this separate guide:
Rules and Regulations
Rules and regulations are issued by the various executive agencies. Proposed and final rules are published in the Federal Register, and then codified in the Code of Federal Regulations.
U.S. Supreme Court
Federal Courts (All other U.S. federal courts, including District, Appellate, and specialized federal courts such as Bankruptcy, Tax, etc.)
"Federal courts hear cases involving the constitutionality of a law, cases involving the laws and treaties of the U.S. ambassadors and public ministers, disputes between two or more states, admiralty law, also known as maritime law, and bankruptcy cases."