LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Free DWG Viewer - Free Viewer for DWG, DXF, DWF View AutoCAD DWG/DWF/DXF files View & measure DWG, DXF, DWF
FreeCAD - An open-source parametric 3D modeler
DXF View - A lightning-fast drawing viewer enables you to view DXF and DWG™ files on Mac
SketchUp - 3D for Everyone
ODA Viewer - ODA Viewer is a web-based software solution that allows you to view the files based on ODA format and you can easily zoom the file for detail visualization.