John the Ripper

John the Ripper is a tool for password cracking and recovery.

How It Works

John works great with other programs, since it does many things well, and each of those capabilities are valuable on their own and can be combined with other programs. For an example, see Aircrack and John the Ripper

First, John has a powerful password generation mechanism. It does not simply take wordlists. It can generate many, many more passwords from those wordlists. For example, starting with a list of the 10,000 most common passwords, John can create additional lists with 400,000 or 8 million additional password variations to try. You can carefully control how those variations are generated to target your password search as you go.

Second, John has support for many encryption mechanisms built-in, so it can handle a wide variety of password files.

References

A fantastic and thorough list of tutorials for beginners: http://openwall.info/wiki/john/tutorials