National Security Archive - collects and publishes declassified documents culled from U.S. government and intelligence sources. Located at George Washington University.
Cryptome - hosts essays, articles, and the text of legislation that covers cryptology, dual-use technologies, national security, and intelligence.
FBI's Electronic Reading Room - displaying frequently-requested documents released under the Freedom of Information Act, including documents related to espionage.
Interception Capabilities 2000 - report that details potential eavesdropping on high frequency radio and microwave relays, subsea cables, and communications satellites; also has a section that addresses the Echelon system. From IPTV.
World Intelligence and Security Agencies - information on the structure and history of intelligence agencies from around the world, from the Federation of American Scientists.
The Complete, Unofficial TEMPEST Information Page - learn more about how with the right antenna and receiver, electromagnetic radiation from computer monitors and other devices can be remotely intercepted, redisplayed, recorded and replayed.