This can be used to crack passwords as well.

Cain and Abel

My personal favourite for password cracking of any kind.

Cain & Abel is a password recovery tool for Microsoft Operating Systems. It
allows easy recovery of various kind of passwords by sniffing the network,
cracking encrypted passwords using Dictionary, Brute-Force and Cryptanalysis
attacks, recording VoIP conversations, decoding scrambled passwords, revealing
password boxes, uncovering cached passwords and analyzing routing protocols.
The program does not exploit any software vulnerabilities or bugs that could
not be fixed with little effort.

Get Cain and Abel Here http://www.oxid.it/cain.html