Free / Open Source
3D Printing Generators
SkeinForge
SkeinForge is an awesome little script that will generate very nice GCode. It's free, it's Python, and it's awesome.
RepRap Host Software
The RepRap host software is now capable of outputting GCode. Its a bit tricky to use, but it does work.
SuperSkein
SuperSkein is a free and open-source Processing program for creating gcode from STLs based around simplicity and hackability. It's also very fast and not prone to slicing errors like Skeinforge.
Slic3r
A fast, multiplatform, multithreaded, easy to configure, free and open source G-code generator supporting STL, AMF, dual extrusion, graphic and command line interfaces, support material, scaling, rotating and much more. Actively maintained and supported in the IRC #slic3r channel on FreeNode.
Make Replicator 2 JSON values
Make Replicator 2 JSON values to "tweak"