RESEARCHING DOCUMENTS IN THE U.S.
The
National Security Archive
probably
the best collection of declassified documents
State
Department FOIA (Freedom of Information Act) site
The U.S. Government's Freedom of Information Act site, fully searchable
by keyword, most documents are available online.
National
Archives Research Room
lots of military and government footage, much (but not by any means all)
of which if public domain, photographs, records, mostly searchable online