BricsCAD - A powerful CAD platform, with features familiar to you from native .dwg applications.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
iCADMac - User friendly 2D/3D CAD software for MAC OS X, which reads and writes DWG format drawing files.
FreeCAD - An open-source parametric 3D modeler
AutoCAD LT - AutoCAD LT is a drafting tool to create 2D drawings.
SketchUp - 3D for Everyone