OS X: Numi is a calculator that can handle multiple operations at once but still give you the answer you intended. For example, if you've wondered what five days from next Tuesday is, or what a 15% discount on a 50 Euro item would be in Australian dollars, Numi can tell you quickly and without you having to do each operation separately.
We've mentioned Numi before, a long time ago, and the app is still great, but it's come a long way since then. It's drastically improved its natural language processing, and it can handle all sorts of different calculations and conversions it wasn't able to do before.
Currency conversions, unit conversions, mathematical functions, date operations -- it understands them all, and can do them in-line with your calculation. You can even tell it basic things like fees and specific variables like the line-item cost of an= product, and have it calculate things like a subtotal or true total with tax or fees. The app even supports CSS units.
Numi is also easy to launch and use once you have it installed. You can open it from Notification Center, from a menubar item, a global hotkey, whatever you prefer. Similarly, the app has day and night modes to fit how you work. The app is free, and the Mac App Store has an older version available, but you can download the much-improved beta (requires Yosemite or El Capitan) below to give it a whirl. Hit the link below to try it out.