Calculator applications are standard on almost every computer and mobile device now, but they can be a pain to use. If you're performing longer equations, it can be really annoying to not be able to see the trail that shows how you got there.
The calculator app included in Mac OS X has a little known feature called Paper Tape that fixes this problem by displaying the history of all of the calculations you made like an old-school tape calculator.
Okay, it's actually better than that.
Activating it is simple. Just press Command T or go to Window -> Show Paper Tape.
The tape can keep track of more than one calculation at a time, and if you mess up, you can go back and correct it or pick up from any previous point. It may not get you through your Calc class, but it definitely makes basic functions a lot less annoying.