FBI Freedom of Information Act Electronic Reading Room - Espionage: Declassified Federal Bureau of Investigation files, in .PDF format, on famous spy cases, including: Anthony Blunt; Burgess, MacLean and Philby; Owen Lattimore; Nathan Silvermaster; Julius and Ethel Rosenberg.
National Counterintelligence Information Center (NACIC): Provides national-level counterintelligence products and services for the US Government and the U.S. private sector.
Of Moles and Molehunters: Former CIA officer Cleveland Cram surveys the literature on Counterintelligence Chief James Jesus Angleton's search for Soviet agents during the 1960s.