Autodesk Design Review - Design Review lets you view, mark up, print, and track changes to 2D and 3D files.
A360 VIEWER - Free online CAD Viewer
ZWCAD Viewer - Cross-Platform .DWG | .DXF | .DWT R12-2013 Viewer
Glovius - Extensible & customizable 3D viewer, for consumption of product design data.
LiteCAD - Create, edit, view, and print 2D CAD drawings using the file formats: dxf, dwg, shp ;Download-Size...
IGES2STEP - Freeware CAD file viewer for Windows XP/2000/NT