Computer-aided Design (CAD)
Includes a list of tools and resources for you to get started on creating and exploring computer-aided designs. Mostly explored its uses in engineering and scientific courses.
Blender
A cross-platform free and open source 3D modelling software. One of the leading software in terms of free software in 3D modelling. It also features video editing and a variety of physics simulation from cloth, smoke, to liquid.
BRL-CAD
A cross-platform free and open source modelling system mainly used for engineering tasks: 3D modelling, image and data processing, and creating diagrams/blueprints.
Personal comment:
Fun fact: This is believed to be the world's oldest software repository which its development started at December 1979. Also, it found its use on the military.
FreeCAD
A cross-platform free and open source modeller. Offers usefulness for various scientific and engineering fields such as electronics, mechatronics, and mathematics.
Google Sketchup
A freemium web-based 3D modelling tool hosted by Google. The free version of the tool is mostly enough to get started on 3D modelling.
KiCAD
A cross-platform free and open source electronics design automation suite.
LeoCAD
A cross-platform free and open source computer-aided design software based on LEGO. If you want to see the source code, you can visit it in GitHub.
Processing
A framework on getting started with programming with visual arts. Mostly used in making generative art.
OpenSCAD
A free and open source CAD where it prides itself being the programmers' CAD. Instead of creating models through a user interface, you have to create a model through scripting. If you want to see the source code, it's in their GitHub repo.
Personal comment:
If you're looking for a start in 3D modelling with free tools, this is one of the de facto tools on it.