Go to any web search engine (Yahoo, Alta Vista, etc.) and search for "Enigma Machine", "Cryptography", "Cipher", "Public Key", "Digital Signature", "National Security Agency", "Bletchley Park", or, if you're not interested in cryptography and just want to have fun, try "Bart Simpson". Below is a small sampling of some webpages worth visiting.
http://www.math.arizona.edu/~rims/workshops/cryptography/cryptolinks.html
Webpage maintained by: Alexander Perlis, aprl@math.arizona.edu