CAM turns a model into toolpaths; the controller turns toolpaths into spindle and axis motion. Open-source covers the whole subtractive stack — from hobby routers to industrial machine retrofits.
The real-time machine controller of choice for serious retrofits — hard real-time motion on commodity PCs.
Integrated CAM inside FreeCAD — model and toolpath in one parametric environment.
The tiny G-code interpreter that runs on an Arduino — the heart of most hobby CNC and laser machines.
Browser-based CAM and slicing — no install, surprisingly capable for 2.5D milling.