The Internet History Sourcebooks Project is a collection of historical texts presented for educational use. It is comprised of Ancient, Medieval, and Modern history sections as well as the following subsidiary sourcebooks: African, Eastern Asian, Global, Indian, Jewish, Islamic, Lesbian/Gay, Science, and Women.
The American Presidency Project archives contains primary source documents related to the study of the Presidency. Includes presidential papers, party platforms, candidates' remarks, Statements of Administration Policy, documents released by the Office of the Press Secretary, and election debates.
Digital library of primary sources in American social history from the antebellum period through reconstruction. This collection is thematically related to the Cornell University Making of America Collection, but includes different materials.
Digital library of primary sources in American social history from the antebellum period through reconstruction. This collection is thematically related to the University of Michigan Making of America Collection, but includes different materials.
The National Security Archive is an organization created by journalists dedicated to a more open government. Among other resources, it includes the largest freely available, non-government library of declassified documents.
The Department of Defense has over 30 FOIA reading rooms, including one for each branch of the military and each unified command. To find individual FOIA rooms, google FOIA Reading room and the agency you're interested in. This link takes you to the Executive Services Directorate Reading Room.
Historically significant documents more than 25 years old are required to be automatically declassified per a 1996 Executive Order. This search includes indexing for all documents that fall into this category, as well as all electronically available declassified documents. Browse a time period or search by name of a person, event, etc. You may get lucky and find some scans, or you may have to submit a FOIA request of your own if you find a document that may be useful to your research. Documents related to overseas intelligence activity will be included.
The FBI's Electronic Reading Room of declassified documents focuses on investigations internal to the U.S., so look here if your topic focuses on events and people within the U.S. rather than relations between countries. See the A-Z index for large collections or search for a country, topic or person using the "search vault" option.