OpenSCAD

OpenSCAD is a CAD tool for creating solid 3D models. OpenSCAD uses a scripting language to describe your model rather than using an interactive editor.

I have a background in software engineering so OpenSCAD is the perfect tool for me. I love the ability to create models that people can easily customise through parameters and Thingiverse customiser supports OpenSCAD.

FreeCAD

FreeCAD is a feature rich CAD tool that can be used to model 3D objects. It is a parametric modeler and comes with an OpenSCAD Workbench for those more familiar with OpenSCAD.

Blender

Blender is a 3D creation suite.

While I've never tried, you can use Blender to create 3D models that you can print.

PrusaSlicer

PrusaSlicer can be used to "slice" your 3D models, generating the code that can be used to print your models.

Whilst there are many slicers available, I found that PrusaSlicer produced the best results when using my CoLiDo Print-Rite DIY 3D Printer.