From the German BSI: Videos like the animation movie about bot networks, edited by the Dutch Government Computer Emergency Response Team (65 MB WMV file):

Webpage to the book "The Code Book" by Simon Singh from the year 2000 and the corresponding cipher challenge:

Website of the Cryptoclub of the University of Illinois at Chicago (UIC):
(very colourful, for kids; sadly, Flash required)

NSA site for kids:

The Spy Camp ("Spioncamp") of the University of Wuppertal offers proved and tested learning stations about cryptography and cryptology for teachers and pupils -- (German only):

Secret Codes and the Enigma Machine (for pupils; site a little bit outdated):

Website about the CODEBREAKERS.EU cryptology game (currently 5th edition):
(e.g. as a game for teams with three members, offering 3 levels a 6 tasks (lectures), supported by hints and "cryptologic bombs" to deal with the Enigma)

Website on the National Cipher Challenge NCC (University of Southampton, IBM, GCHQ):

Famous websites dealing with encrypting and breaking classical ciphers:

Further links at the website of the CrypTool portal (CTP):

Further links at the website of the MysteryTwister C3 cipher challenge (MTC3):