ERIC (via ProQuest)  (MIT only)
Reports and studies from the Educational Research Information Center.

Executive Branch

Compilation of Presidential Documents
Official publications released by the White House, 1993 to date

Federal Agency Directory
A-Z index of US government departments and agencies.

Historic Documents

Core Documents of Our Democracy
Cornerstone documents of American democracy digitized by the US Government Printing Office.

Primary Documents in American History
Digitized historical documents from the Library of Congress collections, 1775-1872.

Judicial Branch

Supreme Court of the United States
Legal information including opinions, court rules, and transcripts of oral arguments.

Law

HeinOnline  (MIT only)
Full text of US Code, Statutes at Large, Code of Federal Regulations, and federal and state case law.

LexisNexis Academic  (MIT only)
Full text of federal statutes and regulations, federal and state case law.

Legislative Branch

Congress.gov
Official site for US federal legislative information. Covers committees, members of Congress, legislation.

Proquest Congressional   (MIT only)
US Congressional publications 1789-present, including full text of legislative histories, Congressional hearings (1824-1979), Congressional hearings testimony (1980-to date),  Data on campaign finance, voting records, and more.

National Security

Declassified Documents Reference System (MIT only)
Declassified documents from various federal government agencies.  1945-2006.

Digital National Security Archive  (MIT only)
Primary documents central to US foreign and military policy since 1945.