Calcbot, by Tapbots, is an easy to use calculator and unit converter. Recently, Tapbots updated the app and is now offering it for free.
The Calcbot 2 update includes style themes to customize your calculator, as well as a new favorites system, the ability to sync your history tape and constants across iOS devices, and an in-app purchase that lets you convert with over 500 units across 22 categories. The app has also been upgraded to work with iOS 8 to make for a smoother experience.
There are a bunch more additions (haha calculator jokes), so check out the newly free Calcbot on the App Store.