Find the product key used during an install of Windows

I have a bunch of machines around here, all installed with licensed versions of Windows. I have a shelf full of boxes with the product keys in them. I needed to know which product key went to each machine. However, Windows is reluctant to tell you you that.

I discovered that if you open a command prompt as an administrator and type:

slmgr /dli

It will give you the last five characters of the product key used to install Windows, as well as tell you which version of Windows you have installed, how it was acquired, and whether it is properly licensed. Unfortunately, it won't give you the complete product key, in case you have misplaced yours. For what I needed, though, it was perfect.