MyScript Calculator (iOS|Android) is an app for both iOS and Android that lets you perform mathematical operations with freehand writing of expressions on your touch screen. This app won the Mobile App Showdown award at CES 2013 and after some time spent using it, it's easy to see why.
The simple-looking interface mimics the look of graph paper. In the top right of the screen you have buttons for undo and redo, and a trash can for when you want to clear the screen completely. In the upper left you can open a menu where you can access the settings with options for setting the number of decimal points the app displays, switching to left-handed input, and other things. But it also has an interesting setting called palm rejection; when turned on, the app will ignore the part of the hand that touches the screen, useful for those who naturally rest their hand on the writing surface when drawing numbers with a stylus.