Blender

BRL-CAD is an open-source, cross-platform solid modeling system that includes interactive geometry editing, high-performance ray-tracing for rendering and geometric analysis, and geometry libraries for application developers.

SolveSpace is an open-source 2D/3D constraint-based parametric computer-aided design software that supports basic 2D and 3D constructive solid geometry modeling.

LibreCAD is an open-source 2D CAD application for Windows, Apple, and Linux. Most of the interface and handle concepts are analogous to AutoCAD, making it easier to use for users with experience in this type of commercial CAD application.

FreeCAD is a general-purpose parametric 3D modeler for CAD, MCAD, CAx, CAE, and PLM, aimed directly at mechanical engineering and product design but also fits a broader range of uses in engineering, such as architecture.

Art of Illusion is an open-source 3D modeling and rendering studio. Many of its capabilities rival those found in commercial programs. Highlights include subdivision surface-based modeling tools, skeleton-based animation, and a graphical language for designing procedural textures and materials.

Clara.io is a web-based 3D modeling, animation, and rendering tool that allows you to create complex 3D models and photorealistic renderings and share them without installing software on your device.

Daz Studio is a 3D scene creation and rendering application for producing 3D models, animations, and more to create your realistic universe.

Tinkercad is a web-based online 3D modeling program for creating models for 3D printing as well as an entry-level introduction to constructive solid geometry in schools.

SculptGL is a web-based 3D modeling and sculpting program. If you want to create a 3D game or video character models that are detailed and unique, SculptGL is a simple and good tool.