ODIC Office of the Director of Central Intelligence.
NACIC National Counterintelligence Center.
NSA Who said there's No Such Agency?
Great jump off page for NSA, et al.
National Security Archive at GWU in DC.
ASIS American Society for Industrial Security
An IW primer.
Intelligence and CounterIntelligence Another decent jump-off point.
Federation of American Scientists intelligence resource program revealing American espionage
Russian intelligence revealed Wow!
JANE'S military-oriented search