There have been a few mentions of cryptography lately in various threads.
For those who enjoy the topic and would like to try their hands at "cracking some code", this site offers a code breaking game: http://www.pc-facile.com/security/codebreak.php
There are 10 levels, going from easy to extremely difficult. It probably helps if you know how to program, although a large portion (maybe all) of the puzzles can be done with various tools you can find on the net.
And yes, the "pookie" sitting at the top of the Hall of Fame (english version) is yours truly. IIRC, it took me about 3 weeks to do them all in August 2004.
It gave me a new respect for Alan Turing and the guys of Bletchley Park who cracked the German Enigma machine during WWII without the benefit of computers. They built mechanical "bombes" who were basically single-purpose mechanical computers who would do brute searches to find the Enigma key codes.