Great software for design communication. Models and drawings can be sent via e-mail and rotated, sectioned, measured on the clients computer.  The viewer only is free.
An evaluation version ( you and open .zip files but can't save them) of a program to open compressed .zip files.
A great way send and receive 2-D mechanical drawings is in Adobe .pdf format.  Get the free reader here.
A good program for viewing a wide variety of CAD formats including .dwg, .dxf, .igs, .stl. The evaluation version has limited file viewing capabilities.
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