Links describes a lot of classical and modern ciphers and geocaching functions in easily understandable words and often offers the possibility to encrypt or decrypt a text online. Many terms from cryptanalysis are also explained:

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)

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Secret Codes and the Enigma Machine (for pupils; site a little bit outdated):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

Great simulations of machines which ran at Bletchley Park and in Germany during WW2:

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 cipher challenge (MTC3):