Two years back I wrote a simple VB program to take a derivative and an integral of a function drawn by hand. It taught me a few things about basic Calculus, and even more about how not to draw sine functions. You can grab a copy of Freederiv and try it for yourself – with a tablet it becomes a great visualization tool. I will provide the source if I can ever find it, but it should be elementary to (re)write. Feel free to do so.
The obligatory screenshot(s):