Mac: Numi is a clever little calculator that lives in your menubar and knows the difference between text and numbers so you can easily add context to your calculations. For example, if you want to make a list of items and how much they cost, then show the total at the bottom of the list with any added tax you could do that just by typing out what you wanted to calculate.
Here's how it would look:
Rolls 12 * $US2 Seaweed Salad $US9 Soda 2 * $US3 Taxed Total: [*sum*] + 8.75%
The sum is calculated by simply pressing Control+Enter at the end of the list. In the example above, you can't see the actual calculations but they appear on the right side of the screen (see the screenshot up top). Basically, you just type out a text file with calculations you want to make and Numi handles the calculations for you. Its exact operation is a little hard to grasp until you actually use it, but it's a free app so all you have to spend is a few minutes of your time. It's very cool and makes simple, common calculations very easy and fun.
Numi (Free) [Mac App Store]