LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Universal Gcode Sender - A full featured gcode platform used for interfacing with advanced CNC controllers like GRBL and TinyG.
FreeCAD - An open-source parametric 3D modeler
bCNC - Development
SketchUp - 3D for Everyone
CNCjs - A web-based interface for CNC milling controller running Grbl, Marlin, Smoothieware, or TinyG. - cncjs/cncjs