From: http://www.bravaviewer.com/free-dwg-viewer

Use the Free DWG Viewer to view AutoCAD files quickly and accurately. The free viewer reads DWG, 
DXF and DWF files without converting them to raster images. Because it supports full vector 
geometry, it offers these great features:

From: http://www.edrawingsviewer.com/ed/edrawings-viewer.htm

Download free eDrawings Viewer software to view, print, and review all types of eDrawings files. 
In addition, eDrawings Viewer allows convenient viewing of supported AutoCAD® DWG and DXF files 
and native SolidWorks® parts, assemblies, and drawings. The eDrawings Viewer is intended primarily 
for people who do not use CAD software and thus do not need to publish eDrawings files themselves.

From: http://www.varicad.com/en/home/products/products/

VariCAD Viewer is a free viewer, convertor and printing software working with 2D DWG, DXF, 3D STEP 
and 2D/3D VariCAD file formats. VariCAD viewer allows you to convert DWG to DXF and vice-versa, 
convert STEP to 3D IGES or STL formats, print 2D DWG, DXF or VariCAD formats and use batch print or 
batch conversions. All conversions are also available between VariCAD native format and other listed 
formats. VariCAD viewer offers settings of 3D display methods (like light source or perspective). 
3D display can be also exported into high-resolution bitmap file.

From: http://www.bentley.com/en-US/Products/Bentley+View/

The infrastructure industry's BIM viewer of choice for interacting with IFC’s, DGNs, DWGs, and 
i-models, as well as point clouds and many other industry formats.
With millions of users, Bentley View is arguably the industry's most powerful free BIM viewer. 
Meeting the industry’s demanding needs for viewing BIM, Bentley View supports over 50 other file 
formats, including CAD and raster image formats.

From:

From: http://www.autodesk.com/products/dwg/viewers#

DWG TrueView Free* download

This desktop software enables you to view .dwg files and includes DWG TrueConvert™ software, 
which lets you convert newer .dwg files to work with older versions of AutoCAD

From:

From: http://www.ida-step.net/components/viewers/basic

Viewer: Basic is a FREE component of IDA-STEP v4 providing core viewing functionality for all kinds of STEP files (*.stp). This component alone makes IDA-STEP v4 a fully usable tool for viewing PDM, CAD and other data, available as STEP (internation standard ISO 10303) files. STEP file format is supported by the major PDM, mechanical CAD and other systems. 

From: http://viewer.kisters.de/en/downloads.html

    3D-Viewer, 3D-CAD-Viewer and 2D-Viewer provided by KISTERS for efficient visualization
    Demo Download - NOT Free

From: https://shop.oracle.com/pls/ostore/product?p1=AutoVue&p2=&p3=&p4=

    Oracle AutoVue - AutoVue Enterprise Visualization is a suite of Oracle solutions designed to view,
    digitally annotate and collaborate on a variety of engineering, CAD and business documents without 
    requiring investment in specialized CAD or authoring tools. Customers can choose from amongst five 
    AutoVue solutions, each designed to suit an organization’s current and evolving document visualization
    requirements.

From: http://kubotek3d.com/Products/Spectrum-Viewer.aspx

    Kubotek Spectrum is low cost, professional-grade CAD viewer software for reviewing 3D models and drawings.
    AutoCAD™, SolidWorks™, and Pro/E™, UG NX, CATIA 4, CATIA 5 (Optional), IGES, STEP and many others.