OpenSCAD

OpenSCAD is a rather unusual option for 3D model design in that it generates models from code. For flexible modelling using a variable driven parametric approach, it is fantastic. There are a variety of models available that will help you generate exactly the output you need for things like Gridfinity.

• openscad.org
• Download - rather than downloading the stable version, the general consensus is to use the latest development snapshot. The stable version is unbearably slow (many minutes) on some renders compared to the development snapshot (mere seconds for the same renders).