LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Autodesk DWG TrueView - Get a free DWG viewer to view, open, edit, and convert .dwg files, the native file format for AutoCAD files. DWG Trueview includes DWG TrueConvert.
FreeCAD - An open-source parametric 3D modeler
LiteCAD - Create, edit, view, and print 2D CAD drawings using the file formats: dxf, dwg, shp ;Download-Size...
SketchUp - 3D for Everyone
ZWCAD Viewer - Cross-Platform .DWG | .DXF | .DWT R12-2013 Viewer