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, here, or here.
For downloads and more information,
visit the John the Ripper homepage.