John the Ripper is a fast password cracker, currently available for many flavors
of Unix (11 are officially supported, not counting different architectures),
DOS, Win32, 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 flavors, supported out of the box are Kerberos/AFS and Windows
NT/2000/XP LM hashes, plus many more with contributed patches.

WWW: http://www.openwall.com/john/