As far as absolutely free cnc operating software, there aren't too many too choose from. I have heard that LinuxCNC is a pretty good software to use. You can download it right from their web site (
www.LinuxCNC.org) and the best part, its free! Now i don't know a whole lot about LinuxCNC, but if its just like all the other Linux based software, there's probably a lot of good info you can dig up just by looking through the site. hope this helps