John the Ripper
John the Ripper is a fast password cracker for UNIX/Linux and Mac OS X.. Its primary purpose is to detect weak Unix passwords, though it supports hashes for many other platforms as well. There is an official free version, a community-enhanced version (with many contributed patches but not as much quality assurance), and an inexpensive pro version. You will probably want to start with some wordlists, which you can find here.
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.
Download one of the latest official free versions:
John the Ripper 1.7.9 (Unix - sources, tar.gz, 848 KB) - Download
John the Ripper 1.7.9 (Unix - sources, tar.bz2, 701 KB) - Download
John the Ripper 1.7.9 (Windows - binaries, ZIP, 2029 KB) -Download