"Grbl is a free, open source, high performance CNC milling controller written in optimized C that will run on a straight Arduino."
-http://dank.bengler.no/-/page/show/5470_grbl?ref=mst
Basically what it allows you to do is use G-code with your arduino for your project whether it would be a CNC milling machine, 3D printer, or anything you want it for. The reason why I picked it is because it looked simple enough and uses arduino and also is inexpensive.