What free gcode generators are available that turn STL files into G-code files for 3D printers?

Before 3D printing your design, it first needs to be converted into a format that the printer can understand. All consumer 3D-printers require ‘G-code’ files, which is an instruction set developed by MIT in the 1950s to control computerised machine tools.

This is a companion discussion topic for the original entry at http://reprage.com/post/47337556946/what-free-gcode-generators-are-available-that-turn-stl