Slic3r is mainly a toolpath generator for 3D printers that can read 3D models (STL, OBJ, AMF, 3MF) and convert them into G-code instructions for 3D printers.
Features:
G-code generation for FFF/FDM printers;
conversion between STL, OBJ, AMF, 3MF, and POV formats;
auto-repair of non-manifold meshes (and ability to re-export them);
SVG export of slices;
built-in USB/serial host controller, supporting multiple simultaneous printers each one with a spool queue;
OctoPrint integration (send to a printer)
built-in projector and host for DLP printers
tool for cutting meshes in multiple solid parts with visual preview (also in batch using a grid)
Blender is a free and Open Source 3D animation suite that supports the entirety of the 3D pipeline—modeling, rigging, animation, simulation, rendering, compositing, motion tracking, even video editing, and game creation.
SketchUp is an easy-to-use web-based program that will help you learn to create 3D models of houses, sheds, decks, home additions, woodworking projects even spaceships.
Tinkercad is a free, easy-to-use app for 3D design, electronics, and coding. It's used by teachers, kids, hobbyists, and designers to design and make anything.