PBS
Nova: Secrets, Lies, and Atomic Spies
The companion site to the show by the same title. Interesting site allows the reader to decipher a coded message. Information on Venona Intercepts and interviews with children of famous spies.
Other
Nat'l Security Agency: National Cryptologic Museum
On this site one can find the National Security Agency's history of cryptology, some online exhibits, and sections on mathematics, education, and more.