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.
Note: a few Windows "antivirus" and "anti-spyware" products have started to recognize password recovery tools as if they were "trojans". This is how those products' vendors inflate their detected "virus" counts. The effect is that end-users are no longer able to check password recovery software forreal viruses. If this affects you, please complain to your antivirus vendor.