LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
DXF View - A lightning-fast drawing viewer enables you to view DXF and DWG™ files on Mac
FreeCAD - An open-source parametric 3D modeler
Free DWG Viewer - Free Viewer for DWG, DXF, DWF View AutoCAD DWG/DWF/DXF files View & measure DWG, DXF, DWF
SketchUp - 3D for Everyone
Amethyst CADwizz Ultra - Amethyst CADwizz Ultra is an AutoCAD viewer software solution that allows you to view the files which are created by AutoCAD software and you can even view the files without the presence of AutoCAD.