Slic3r

The Slic3r project was born in 2011 within the RepRap community as an effort to provide the growing 3D printing technology with an open and flexible toolchain. The code and the algorithms are not based on any other previous work. Readability and maintainability of the code are among the design goals. Slic3r, being a true non-profit community project, allowed the people to experiment with several original new features that have become common thereafter such as multiple extruders, brim, microlayering, bridge detection, command line slicing, variable layer heights, sequential printing (one object at time), honeycomb infill, mesh cutting, object splitting into parts, AMF support, avoid crossing perimeters, distinct extrusion widths, modifiers, and much more.

All of these features were first introduced in Slic3r and are now part of the commercial software out there.