ECam - Nice, fast and intuitive. A 2D Cad/Cam designed for simple machining operation
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
FreeCAD - An open-source parametric 3D modeler
PyCAM - PyCAM is a toolpath generator for 3-axis CNC machining.
nanoCAD - nanoCAD is a pro-grade 2D CAD system with native DWG support and full customizability.
LinuxCNC (the Enhanced Machine Control) - LinuxCNC: an open source CNC machine controller. It can drive milling machines, lathes, 3d printers, laser cutters, plasma cutters, robot arms, hexapods, and more.