Slic3r converts digital 3D models into printing instructions (G-code) for your 3D printer. It cuts the model into horizontal slices (layers), generates toolpaths to fill them and calculates the amount of material to be extruded.
Slic3r supports input in the STL, AMF and OBJ formats, and can output G-code for several series of 3D printers, including RepRap, Ultimaker, Makerbot, as well as SVG files for DLP printers.
It can be used with a graphical interface, or in batch mode via the command-line.