Tinkercad
Tinkercad is a free, easy-to-use web app that equips the next generation of designers and engineers with the foundational skills for 3D design and printing.
TinkerCad is a great launching point for people of all ages to get into 3D design. Even though it is a simpler CAD program, it is very powerful because it makes it quite simple to make functional parts. By grouping shapes and cutting holes, you can make fairly complex shapes that measure out to the millimeter.
Supported filetypes
STL (best for 3D printing), OBJ, GLTF, USDZ, SVG, DXF
Minimum system requirements
Up date web browser capable of running WebGL (Firefox, Chrome, Safari). At least 1GB of RAM. At least 500 MB of available hard-disk space. Intel HD or similar graphics card with at least 512 MB video memory