KiCad: A cross-platform, Open Source suite for electronics design automation that effectively manages schematic capture and PCB (printed circuit board) layout. MajorGeeks also has the portable version.

SketchUp is an easy-to-use web-based program that will help you learn to create 3D models of houses, sheds, decks, home additions, woodworking projects even spaceships.

LeoCAD is a free app to design virtual models that you can build with LEGO bricks.

TopSpice is an integrated electronic circuit design system with multiple capabilities, including schematic capture front-end, mixed mode simulator, graphical representation, and library of over 35,000 device models.

FreeCAD is a parametric 3D modeler for CAD, MCAD, CAx, CAE, and PLM aimed directly at mechanical engineering and product design.

KiCad Portable is a robust, cross-platform Open Source suite designed for electronics design automation. It effectively handles both schematic capture and printed circuit board (PCB) layout, catering to a variety of design requirements. It supports multiple output formats, including Gerber files, making it a versatile choice for designers. Installer, Linux, and Mac versions are also available.

LibreCAD Portable is a user-friendly graphics suite for use on 2D CAD (computer-aided drafting) projects. Installer, Linux, and Mac versions are also available.

OpenSCAD is software for creating solid 3D CAD objects.

gCAD3D is a free CAD app that provides the tools needed to create expertly rendered 3D models.

Tinkercad is a free, easy-to-use app for 3D design, electronics, and coding. It's used by teachers, kids, hobbyists, and designers to design and make anything.

XTrackCAD is a CAD program for designing model railroad layouts. You can easily create a layout of any scale or size.