Ophcrack is a Windows password cracker based on rainbow tables. It is a very efficient implementation of rainbow tables done by the inventors of the method. It comes with a GTK+ Graphical User Interface and runs on Windows, Mac OS X (Intel CPU) as well as on Linux.
There is also article and more screen shots at Lifehacker:
Extremely impressed at the ease and speed with which the Ophcrack Live CD cracked my Windows admin password when I tested it out last a few weeks ago, I thought it might be useful to throw together a quick guide detailing how to use this powerful little utility.
Whether you need to recover the lost password to a Windows account, you're looking to ensure that your passwords are secure, or you're a super l33t h4x0r, the Ophcrack Live CD is a pretty useful tool.