This tool can be used as a tool to help you decode many encryption methods. It won't do all the work for you, but can easily decode ROT13 codes, and help you in breaking substitution ciphers.
It is very useful for decoding some of the messages found while Geocaching!
For a good example of how to go about cracking a password, why not visit my How and Why I cracked a password hash.
I learnt most of my code breaking skills from a book called The Code Book: The Secret History of Codes and Code-breaking. It's a fantastic book by Simon Singh and I highly recommend it.
NOTE: If you're after a ROT decoder, please visit my ROT13 encoder and decoder page.