This is the firmware we use for the Lasersaur. It's a slightly modified version of grbl. It's what runs on an Arduino Uno and takes g-code files to controls the stepper motors accordingly.
How to get this firmware onto an Arduino Uno? There is a makefile that will do the trick. Edit the Makefile and follow the instruction in it. You will need a USB cable and the Arduino IDE.
For more information on Grbl