Calculators have always been a hard thing to get right in the app world, or wrong depending on which way you look at it. The majority of apps available don't really differ from the one we could just get out our desk drawer, then there's Numi.
Numi is a minimal Mac calculator app that breaks away from the traditional numbered keypad, and instead aims for more text-based equations that let you see exactly what is being added and subtracted.
Numi looks more like a text-editor than a calculator, which in effect is its selling point - as you type it scans for equations and creates totals on the right-hand side.
It's currently in its beta stage, which you can download here.