Of Itself So - A Summary of Gerber to GCode Isolation Routing Software
 
"... and no one shall work for money, and no one shall work for fame; But each for the joy of the working, and each, in his separate star, shall draw the thing as he sees it, for the god of things as they are"
This is a free, downloadable ULP for the well known Eagle circuit board software. The PCB-GCode software is free but a full version of Eagle is necessary for PCB boards larger than 3 x 3 inches.
According to the wiki: pcb2gcode is a command-line program that calculates machine toolpaths from given PCB layouts. It is free and open-source and runs on Linux. Requires things like libgerbv to be installed.
From the web page: Among many features, supports importing of RS274X (Gerber) and Excellon drill files for gerber to g-code PCB milling conversion. It is free and open source.
From the web page: This project is for converting PCB RS274X gerber files to RS274NGC GCode files. Currently this software is in the early development stages.
From the web page: This is an open-source freeware which open Gerber, Excellon or G-code, edit it or create from scatch, and output G-Code Isolation routing files. Offers both a Windows and Linux download.
From the web page: The Line Grinder is a software package which can generate isolation routing GCode from the Gerber format plot files output by most PCB design programs. It runs on Windows, is free and open source.
From the web page: Open source no frills Gerber to gcode converter, using (a slightly modified) Clipper Lib. Produces an isolation routing gcode file for the given Gerber file. GPLv5 License.
Have I missed any? If so, let me know, and I will update this page.