Congressional Record up to 1875
For a well-designed site featuring 100 key US documents, see OurDocuments.gov
Government Printing Office: FDsys
Government Documents online, via the Internet Archive
Constitution of the United States – see virtual exhibit on the National Archives website
State Constitutions (University of Maryland Baltimore)
US Serials Set (some of which are online, some of which are only in bound volumes in depository libraries) http://www.llsdc.org/serial-set-volumes-guide#Overview
Checklist of Government Publications, 1789-1909
Understanding Su Doc Classification Numbers: an online tutorial (Michigan State U)
Congressional hearings, via the Library of Congress
See also Senate hearings on the US Senate website
Federal Register (National Archives, since 1994)
Supreme Court cases: see the US Supreme Court website, also the Oyez Project
National Security Archive – a massive document dump of declassified material from FOIA requests (George Washington University)
Native American Tribal Law, Constitutions, and Treaties – National Indian Law Library
Miller Center for Presidential Studies (University of Virginia)
American Presidency Project (Univ of California Santa Barbara)
Many 20th-century presidents have Presidential Libraries: e.g. Nixon, Truman, FDR, Hoover
Massachusetts State –
Massachusetts Archive Collection
MA Historical Legal Documents and Laws