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.
Perfect for math students or anyone who wants to make a quick calculation, MyScript Calculator converts hand-written numbers and symbols into clear notation before your eyes, then shows you the solution to a math expression instantaneously. Not only can you add, subtract, multiply, and divide; you also can solve trigonometric functions by writing sin, cos, tan, and the degree of the angle on the screen. The app supports surprisingly complex expressions, letting you add exponents, draw the radical symbol for square roots, draw a line to divide by another number, and so on. You can continue to add to each mathematical expression or erase parts of it until you get it exactly right.