Try, Slic3r a this link http://slic3r.org/
its a fast, multiplatform, multithreaded, easy to configure, G-code generator.
it also supports STL, AMF, dual extrusion, graphic and command line interfaces, support material, scaling, rotating etc. Actively maintained and supported in the IRC #slic3r channel on FreeNode.
and yes they are 100% free!
you can also try the following aswell;
RepRap Host Software