The Roosevelt Study Center
A research institute, conference center, and library on modern American history.
The Avalon Project; Documents in Law, History and Diplomacy
One of the best internet resources for primary documents, collected, annotated and kept up-to-date by Yale Law Department.
Internet Modern History Sourcebook
A huge collection of primary sources on a range of topics, part of the Internet History Sourcebook projects at http://www.fordham.edu/halsall/. Not very structured and not always up-to-date, but a lot of valuable sources.
Vietnam War bibliography by Edwin Moïse
The most successful effort to collect and categorize all publications on the Vietnam War. The site is constantly being updated.
Library of Congress
A portal to many more sub-sites with high quality resources on all aspects of American politics, culture and history.
Thomas Library of the Library of Congress
Federal legislative information: Bills, Resolutions, Votes, etc
American Folklife Centre
For resources on American cultural life (in a very broad sense).
National Archives of the United States, with links to all the websites of the Presidential Libraries
Digital National Security Archive
A site with declassified documents requested through the Freedom of Information Act. The subscribers area has well-prepared collections on a range of topics, but the free area also has valuable sources.
State Department FRUS-series
The Foreign Relations of the United States (FRUS) series contain the official historical record of US diplomacy. Recent publications available in pdf.
Congressional Record (1994-Present)
Constantly updated online archive of the Congressional Record.
Frontline Diplomacy: Foreign Affairs Oral History Collection
Transcripts of hundreds of interviews with US foreign service personnel serving all over the world. A treasure-trove of insights and information from those who worked at the front line of US diplomacy over the past decades.
The US-Mexican War
Detailed information on the 1846-1848 US-Mexican war, including a vast collection of historical documents, biographies, maps, and teaching tools.