National Archives [website]--the National Archives and Records Administration (NARA) is the nation's record-keeper. Of all documents and materials created in the course of business conducted by the United States federal government, only 1 to 3 per cent are so important for legal or historical reasons that they are kept here for you to search and access.
National Security Archives: Virtual Reading Room [website] founded by journalists and scholars, this is the world's largest nongovernmental collection of declassified U.S. government documents.
Wilson Center Digital Archive [website]--"contains once-secret documents from governments all across the globe, uncovering new sources and providing fresh insights into the history of international relations and diplomacy"--From the Archive's homepage.