C? CE? I can never remember what each one does. So when I need to clear the current calculation, I hit them both (a couple of times actually... you can never over-erase a current calculation as far as I'm concerned... same goes for crosswalk buttons, but that's a different story).
So today while calculating income from a the few little projects that I've patched together for the summer (ah, academia), I had to choose CE or C. On a hunch, I right mouse clicked over the buttons and was delighted to see, "What's this?" followed by an explanation.
The mouseover help not only tells me what each button does, but explains it in terms of other familiar computer functionality. (Well, who uses the ESC key? But it's a nice gesture.) Now I'll just have to tape that information on my manual calculator.
[Update: I'm not the only one on about minute levels of usability today.]