Universal Gcode Sender - A full featured gcode platform used for interfacing with advanced CNC controllers like GRBL and TinyG.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
bCNC - Development
FreeCAD - An open-source parametric 3D modeler
Candle - GRBL controller application with G-Code visualizer written in Qt
SketchUp - 3D for Everyone