John the Ripper password cracker

John the Ripper is free and Open Source software, distributed primarily in source code form. If you would rather use a commercial product tailored for your specific operating system, please consider John the Ripper Pro, which is distributed primarily in the form of "native" packages for the target operating systems and in general is meant to be easier to install and use while delivering optimal performance.
Proceed to John the Ripper Pro homepage for your OS:

HOW DOES IT WORK??
John the Ripper is a fast password cracker, currently available for many flavors of Unix, Windows, DOS, BeOS, and OpenVMS. Its primary purpose is to detect weak Unix passwords. Besides several crypt(3) password hash types most commonly found on various Unix systems, supported out of the box are Windows LM hashes, plus many more with contributed patches.
Download one of the latest free versions (and donate to the project):   

• John the Ripper 1.7.5 (development, Unix - sources, tar.gz, 808 KB) and its signature
• John the Ripper 1.7.5 (development, Unix - sources, tar.bz2, 650 KB) and its signature
• John the Ripper 1.7.3.4 (stable, Unix - sources, tar.gz, 798 KB) and its signature
• John the Ripper 1.7.3.4 (stable, Unix - sources, tar.bz2, 642 KB) and its signature 

FOR WINDOWS:

• John the Ripper 1.7.0.1 (stable, Windows - binaries, ZIP, 1360 KB) and its signature
• John the Ripper 1.7.0.1 (stable, DOS - binaries, ZIP, 895 KB) and its signature