KiCad

KiCad is a cross-platform, free and open-source collection of software for electronic design automation (EDA).

KiCad's integrated development environment (IDE) focuses on tools for schematic capture, PCB layout, and generating files from them in the standard for manufacturing PCBs -- Gerber format and Excellon drill format. It also has tools to create a bill of materials (BOM), do design rule checking (DRC), view Gerber files, view 3D models of the PCB and its components, run SPICE simulation, etc.

Unlike most CAD software (and, more specifically, unlike most EDA software), which save files in some unreadable binary format, KiCad's file formats are more-or-less human-readable UTF-8 text formats.

As of 2022, current versions of KiCad produce files with these file extensions:
 * ".kicad_sch": a schematic sheet, typically containing many symbols and the connections between them.
 * ".kicad_sym": ("symbol") single-symbol (or multi-symbol?) library
 * ".kicad_pcb": an entire PCB layout, generally containing many footprints and the connections between them
 * ".kicad_mod": ("module") single-footprint library
 * ".kicad_wks": work sheet file

Some pre-2014 versions of KiCad produced files with these file extensions:
 * ".sch": schematic sheet
 * ".lib": schematic symbol
 * ".brd": ("board"): an entire PCB layout

Other EDA software comparable to KiCad: gEDA EAGLE OrCAD

Other kinds of CAD software that, like KiCad, store files in more-or-less human-readable text: OpenSCAD