I bought a keyboard for a friend recently and I was suprised to see it had an extra light on it:

I couldn't get the windows software to install on the only windows machine I have with a keyboard, so I don't know what it's for. - To cut a very long story short I wrote some linux code which will turn the coffee led on this keyboard on and off. I can't imagine a use for it at all ever, but there you go.

[root@heidh coffee]# coffee on
Coffee led is now on
[root@heidh coffee]# coffee off
Coffee led is now off
[root@heidh coffee]#

Yes I need to make my coffee led flash give me the files