If you want 'free as in beer' - not too sure. QCad by itself is free and open. If you want tools for CAM the same people offer up CAMexpert http://www.qcad.org/index.php3?product=cxp
It's a commercial package, but compared to the price we paid for similar software only ten years ago you could pay for it with the change you find in the sofa.
I trust you already have the machines wired up for receipt of the code - i'd hate to see a 'G' replaced with an 'M' due to severe line noise. If noise is a problem you can always use a 20mA current loop and convert to plain old serial on the ends for serial. Or give that IBM spec cable (...forget the name now) twisted pair with shielding - for ethernet.
(BTW - I used to be a CNC nerd about *yeesh* ten plus years ago.)