No one mentioned SolveSpace, so… SolveSpace. Solvespace is a fully functional 3D parametric CAD solver in a free, cross-platform, open-source, portable, single self-contained executable 10 MB file.
I do a fair amount of hobby 3d printing and SolveSpace makes design and CAD stupid easy. The interface is perfectly laid out, the hotkeys are intuitive, and the capabilities make small-scale projects a breeze.
Now, the program has its limitations, but if I just want something quick and simple, there is nothing better.
Yeah, FreeCAD is great, but I can only think of one project that I’ve done where SolveSpace absolutely could not work for the geometry I needed and I had to rebuild it in FreeCAD. But that’s just a product of what I am building: simple things like brackets, knobs, and replacement lids mostly. I don’t need chamfers, drafts, lofts, etc. and I get what I need with minimum of time and effort using SolveSpace.
When I do need those features, complex geometries, or modification of pre-built step files, FreeCAD has never failed me.