pcb2gcode is a command-line tool for isolation routing and drilling of PCBs that provides full support for both single- and double-sided boards. For more information, see http://sourceforge.net/apps/mediawiki/pcb2gcode/
Best gerber->gcode coverter i came across. Sad to see end off active development, but there isn't much do develop, Multi pass engraving maybe...
This software produces a more efficient tool path than the gcode exporter built in to the latest gEDA
This is recomended software for anyone to easy undestand about CNC