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What is the best free product design CAD software on the market?
What do you mean by product design?
I'm thinking about coffee machines, shoes, alarm clocks, ....
Many product designers are using Modo (not free) for preliminary
design, prototyping, variations and photorealistic presentations
of such products.
Blender (free) is pretty close to Modo.
Both are 3D Polygon Modelers and no CAD though.
In jewelry design it is often Rhino (primary Nurbs Modeler, not free)
+ VRAY Renderer (not free)
If you want to go totally free and open source I would go with Linux
and check Gimp ( 2D Pixel), Krita and Inkscape (2D Vector),
Blender (3D Polygons) and FreeCAD (2D/3D/BIM CAD)
While FreeCAD is still at its beginning, many use it in production.
And if "Products" means more hard-surface models similar to buildings,
also try Bricscad Shape (free)