FreeCAD - An open-source parametric 3D modeler
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.
QCAD - QCAD is an application for computer aided drafting in 2D.
Autodesk Design Review - Design Review lets you view, mark up, print, and track changes to 2D and 3D files.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
DWG FastView - DWG FastView for Windows is a lightweight dwg/dxf drawing viewer and editor produced by Gstarsoft, no need of dwg format conversion, vector graph magnified without distortion and full compatible with AutoCAD DWG/DXF.