GRBL
From PROBOTIX :: wiki
GRBL is an open source 4-axis CNC controller that runs on Arduino compatibles microcontrollers. You command the machine to move by sending g-code over a serial port. There are GUIs that aid in the control of the machine that run on PC, Mac, and Linux. GRBL is the least user friendly of the CNC control options, but it is well documented and has a large user base.
While GRBL runs on Arduino compatible microcontrollers and can be programmed using the Arduino software platform, it makes use of every inch or realestate in the microcontrollers program memory space, including the Arduino bootloaders memory space. This means that you need an Atmel programmer such as the Pololu 1300 to install the firmware on the controller.
Adding the Pololu 1300 programmer to the Arduino software