Calculator app
You might not use a calculator app much, but we have to take this opportunity to give a mention to PCalc (iOS): It features just about every tool and option you could possibly think of for a calculator app and then some, and it’s well worth the $10 price of admission.
There’s a vast choice when it comes to layouts, inputs, outputs and numeric systems, and everyone from engineers to students is going to get a whole lot of use out of this. Sadly PCalc isn’t available for Android, but HiEdu Scientific Calculator (Android) is also great.
