FreeCAD

FreeCAD is a parametric 3D modeller: a part is built as a sequence of operations, each with parameters you can go back and edit, so changing a sketch dimension or the depth of a pocket recomputes everything downstream instead of forcing a redraw. Geometry comes from the OpenCASCADE kernel, which works with exact solid surfaces rather than meshes.

The program is divided into workbenches you switch between as the job changes. Sketcher draws 2D profiles held together by geometric and dimensional constraints. Part Design turns those profiles into features such as pads, pockets and fillets. TechDraw produces dimensioned drawings from a finished model, Spreadsheet holds values other parameters refer to, FEM prepares finite element analyses, and CAM generates machine toolpaths.

Everything is scriptable from an embedded Python console, and macros and community workbenches install through the add-on manager. STEP and IGES handle exchange with other CAD systems, alongside STL, DXF and SVG. Worth knowing: the workbenches differ noticeably in maturity, so the experience is not uniform.