Currently we use Offline NT Password & Registry Editor for machines we do not have admin passwords for. It is a really nice tool but has some flaws. Is there something better/more modern?
I've had success with OphCrack
Before it was pulled, l0phtcrack was the way to go. L0pht went commercial and the tool was pulled. ophcrack is a "branch" of this and will provide you with what you need (see Kenvin Dente's answer for linky).
This may or may not apply to your situation, but to avoid this issue, the Local Administrator account is set to a different password, and is never given to anyone for any reason. So, even if the machine is disconnected from the network, and you've used up your 15 cached attempts (the default), you can still get in and affect changes as needed. Users that need special privs (when it's absolutely required because the program's concept of security is botched) we grant them "Power User", etc.