Linux is probably the most successful open-source project, in terms of its scale, quality, and wide adoption. I think KiCad is on a similar trajectory, though in the much smaller niche of electronics CAD.
Commercial programs might look a bit nicer and have some extra bells and whistles, but KiCad offers everything needed to get to a quite sophisticated PCB. Circuit simulation is possible, but requires some legwork to find accurate models of specific components (unlike LTSpice, which comes with a substantial library). However, KiCad’s built-in libraries for schematic symbols, PCB footprints, and 3d component models are quite comprehensive; if you have an exotic part that’s not already in them, you can recreate all but the 3d model easily in KiCad itself.
If your plans stop at drawing a basic schematic or simulating the circuit, Falstad or LTSpice might get the job done with less learning curve.
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