More specifically, the Offline NT Password and Registry Editor tool, located under NTFS File System Tools of the Ultimate Boot CD. 

We have a machine here at the office sitting in an empty cube that we use for a variety of reasons.  Unfortunately, our infrastructure team has a utility that goes out and checks all of the machines on the domain to see if they've been used recently.  If they haven't been used in a certain period of time they're booted off the domain, which is what happened to our utility machine here.  So we call the help desk to get the computer back on the network and we begin the usual process.  Since the machine has been taken off the domain, you can only login using the local administrator account that every company-owned machine is built with.  Now the problem is that because of additional security measures, the local administrator password is changed periodically.  Which means that depending on when the machine was built, the local admin account will have a different password.  The solution for this is that you call the help desk and they read off a dozen passwords until you find one that works.

Unfortunately for us, today, none of the passwords worked.  Open source software to the rescue!  I booted up the Ultimate Boot CD and ran the Offline NT Password utility.  It recognized the hard disks (even though they were in a RAID array :).  I then browsed to the appropriate SAM hive and it listed all of the users on the machine.  I selected the local administrator and blanked out the password.  Saved the changes, rebooted the machine, and logged into Windows as though nothing ever happened.  Sweet.