Slic3r is free software 3D slicing engine for 3D printers. It generates G-code from 3D CAD files (STL or OBJ). Once finished, an appropriate G-code file for the production of the 3D modeled part or object is sent to the 3D printer for the manufacturing of a physical object. As of 2013, about half of the 3D printers tested by Make Magazine supported Slic3r.
Slic3r is free software without any restrictions. Plus, you can’t find ads, trackers and so forth which makes it completely trade-free.