Autodesk Design Review - Design Review lets you view, mark up, print, and track changes to 2D and 3D files.
OpenSCAD - OpenSCAD is a software for creating solid 3D CAD objects.
LiteCAD - Create, edit, view, and print 2D CAD drawings using the file formats: dxf, dwg, shp ;Download-Size...
FreeCAD - An open-source parametric 3D modeler
ZWCAD Viewer - Cross-Platform .DWG | .DXF | .DWT R12-2013 Viewer
ImplicitCAD - ImplicitCAD is a programmatic CAD program, implemented in Haskell.