active password cracking tool

 John the Ripper is a tool designed to help systems administrators to
 find weak (easy to guess or crack through brute force) passwords, and
 even automatically mail users warning them about it, if it is desired.
 .
 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/2003 LM hashes, plus several more with contributed
 patches.