Diplomatic and Military History
The mother of all diplomatic history websites, with links to everything you can imagine (and some things you can't)
Assorted CIA analyses of the Soviet Union, 1947-91
The most comprehensive list of secondary sources
Collections from the National Security Agency's Venona Program, which intercepted Soviet communications during the Cold War
The Society for Historians of American Foreign Relations
Great collection of WWII materials, especially related to Pearl Harbor
Society for Military History, with links to documents, the "Journal of Military History", and other useful connections
Various NSA documents about cryptology and the Cold War
Various military intelligence documents and links
A collection of Vietnam War documents with a focus on American military strategy
Foreign Affairs Oral History Collection of the Association for Diplomatic Studies Assorted World War I documents
Various strategic intelligence links and documents
Assorted collection of Vietnam War documents and oral histories
Documents in American law and diplomacy
Great material about the Hiroshima bombing including documents, excerpts from diaries and personal papers, and many relevant articles
Home of the Cold War International History project, with on-line bulletins and documents
Basic information from the CIA about the countries of the world
A great collection of State Dept. documents starting in 1861
The United Nations website
The National Security Archive with many good documents post-1945
Assorted foreign policy documents at Mt. Holyoke College