Blender is a 3D computer graphics software toolset used for creating animated films, visual effects, art, 3D printed models, interactive 3D applications and video games. Their showcase (https://www.blender.org/features/) demonstrates the advanced capabilities of this tool, which supports the entire 3D pipeline: modeling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation. Blender runs on Windows, Mac, and Linux, and can serve as a free alternative to Maya and 3DS Max.
Take your Lego game to the next level with BrickLink Studio. This program will allow you to fully assemble custom Lego projects with photorealistic rendering, stability checking, and an integrated instructions builder. Completed designs can be exported and ordered online. For more customization, BrickLink offers the complementary PartDesigner, which exports to Studio and can be used to create custom pieces and decals. Runs on Mac and Windows.
FreeCAD is an "open-source parametric 3D modeler made primarily to design real-life objects of any size." With a thriving community, FreeCAD has plentiful tutorials to get you started. It works on Windows, Mac, and Linux, and can serve as a free alternative to AutoCAD.
AutoDesk Fusion 360 is cloud-based platform with tools for 3D CAD, CAM, and CAE. It is an industry standard paid program that is free for educators—a great deal for those in school. It works with Mac and PC.
MagicaVoxel is “A free lightweight 8-bit voxel editor and interactive path tracing renderer.” It can be used to create striking isometric models out of the 3D equivalent of a pixel, which is known as a voxel (volumetric pixel). With the help of a robust community and plenty of tutorials, even a beginner can create striking voxel art. The models made with MagicaVoxel can be used in games and animations. It runs on Windows and Mac.
MakeHuman is a 3D computer graphics software middleware designed for the prototyping of photo realistic humanoids. It can be used (typically in conjunction with another modeling software) to make 3D humans for games, animations, and other purposes. It works with Windows , Mac, and Debian/Ubuntu derivatives.
MeshLab provides a set of tools for working with meshes. It offers features for processing raw data produced by 3D digitization tools/devices and for preparing models for 3D printing. It works with Windows, Mac, and Linux.
Sketchup Free is the most basic version of the paid program SketchUp. Advanced plans are popular for 3D printing and working, as well as architectural, interior design, landscape, civil and mechanical engineering, film and video game design applications. SketchUp Free is web based and has a much lighter set of features than the paid versions, but is very suitable for those getting started in 3D Design.
Gimp (GNU Image Manipulation Program) is often thought of as a free version of Photoshop. This open source program is used for image retouching and editing, free-form drawing, converting between different image formats, and many other tasks. It works with Windows, Mac, and Linux.
Inkscape is a free and open source vector graphics editor that can be used in place of Adobe Illustrator. It supports a number of file formats, and can be used to create diagrams, illustrations, logos, and webgraphics. It works with Windows, Mac, and Linux.
Krita is a free and open source painting program. It is considered an alternative to Autodesk Sketchbook. It is helpfully designed with a tablet user in mind, minimizing text navigation. Their gallery gives an idea of the impressive capabilities of this program. It works on Windows, Mac, and Linux.
Medibang Paint is a popular digital painting and comic creation tool. It is specifically designed for manga artists and includes features like easy panel creation and page management. It works with Windows and Mac.
While you might not know the name OpenToonz, you certainly know some of the animation it has been used to create. Originally called Toonz, OpenToonz has been used to for Ghibli films and popular animation shows like Steven Universe. This free and open source animation program works on Windows and Mac, and is suitable for professional animation projects.
Godot is cross-platform, free and open-source game engine. It can be used to make 2D and 3D games for PC, mobile, and web. It is considered easy to use for non-programmers and great place to get started in game design. Godot works with Windows, Mac, and Linux.
Solarus is "a lightweight, free and open-source game engine for Action-RPGs." It is a game engine made specifically for making Zelda clones, and many games are available on their website. It is similar to the popular RPGMaker series. It is available on many platforms, including Windows, Mac, and Linux. Games require Solarus to play.
Unity is a well respected game engine that has a free personal edition. Unity aims to democratize game development, and has been used to create many well known games, including Untitled Goose Game. It is also used outside the gaming industry in the automotive and film industry. The editor works with Windows and Mac, and games are playable on many platforms.