Electronic Briefing Books, National Security Archive, The George Washington University
"Provide timely online access to critical declassified records on issues including U.S. national security, foreign policy, diplomatic and military history, intelligence policy, and much more."
CREST: 25-Year Program Archive, (the CIA Records Search Tool)
"Publicly accessible repository of the subset of CIA records reviewed under the 25-year program in electronic format (manually reviewed and released records are accessioned directly into the National Archives in their original format)."
Other Free Resources for Finding Declassified Documents, Library of Congress