Wow. That's so cool. I was going to vote for ScrollLock but read the replies before voting. I'm glad I did. This sounds like a key I'll have much use for, if the things I use actually support it.
I've used PrtScr several times in the last few weeks (it stuffs the current screen contents into the paste buffer so I can crop them and include them in e-mail to managers who like their facts in the form of pretty pictures). :)
Just for the sake of tradition, I'll note that the best answer was omitted. NumbLock, which should always be "on" (unless you have a miniature keyboard as part of some miniature device, of course).