There are alot of utilities for Windows. Try this one http://pogostick.net/~pnh/ntpasswd/
of course, this is a password reset option and not password retrieval/cracking. But the end result, getting the users account with physical access is there... only they'll wonder wtf they can't login the next time they come back when their password isn't working.
There is also something called password genius http://www.pcworld.com/downloads/file/fid,112039-order,4/description.html
i haven't used it myself but I have a house-call tech support friend who swears by it... probably paid version.
I'll second the fuck mac opionion from above.
Linux could potentially be different approaches based on the distro, for Ubuntu you can easily reset the root pwd with a Live CD or Live USB. Once again I've only given a reset option, I'm sure there is a recovery option as well though.. for all the OS's as long as you have physical access to the machine.