See Really Large Results in Calculator
Have you ever thought that the results display in the built-in OS X Calculator app was too small? No, I haven't either...but perhaps, just perhaps, one day you may want to see the results of a calculation in a much bolder, larger, and easily-read-from-across-the-room form. If so, look no further than your handy Control key and mouse button (or if you're multi-button-mouse-enabled, the right mouse button).
After performing an operation in Calculator, Control-click (or right-click) on the results section of the display, and select Large Type from the pop-up menu (also notice that you can enable or disable RPN mode via the same pop-up menu).
Just like that, you'll see the final result of your operation in large white type (against a translucent black background) in the center of your screen.
Now, I'm not sure exactly when you might find this necessary, given you have to be sitting at the machine in order to use the mouse to see the large results--perhaps you're hosting a party, and a guest across the room has yelled out if anyone knows the 8th root of 1,500,000. With a few keyboard taps and a mouse click, you can show the guest--and everyone at the party--that the answer is 5.915771726 (but I'm sure you knew that already), all without raising your voice.
OK, so that's a contrived example...but I'm hard pressed to come up with a real-world need for large calculator results. (I assume those with vision impairments will already be using Universal Access to improve the legibility of the entire display.) Nonetheless, if you ever need this power, now you know it exists.