KiCad

Free tiers of PCB software usually hide a wall: two layers only, boards under 10 cm, no export past 100 components, licence required for commercial use. KiCad has none of them — GPL-3.0, and designing a product you intend to sell costs nothing.

The workflow is one unbroken chain: draw the circuit in the schematic editor, assign real footprints to each component, lay out and route in the PCB editor, check the assembly in the 3D viewer (exportable for mechanical fit checks), then generate Gerber and drill files for the fabricator. Not one step in that chain requires payment or a different program.

Recent releases improved enormously — push-and-shove routing, differential pairs, interactive length matching and multi-channel design were all features once exclusive to commercial tools, and the interface bears little resemblance to the "usable but painful" versions of years past. Many open hardware projects and small production runs now use it as their primary tool.

Worth noting that CERN has invested in it long-term, with their engineers building features for their own instrumentation needs and contributing them back. That makes it far less exposed to the abandonment risk that shadows many niche open-source projects.

A tip for starting out: the relationship between symbol libraries and footprint libraries is where beginners get lost. The schematic symbol (what it looks like) and the PCB footprint (how much space it takes and where the pads are) are two separate libraries you associate manually. Once that clicks, the rest of the flow follows.

Note also that the source repository lives on GitLab rather than GitHub; download from the official kicad.org.