looking for an open source CAD program to at least be able to open a .dwg file
Check LibreCad: http://librecad.org/cms/home.html
Though I am not quite sure on its dwg support state
What comes to mind is Sketchup and DraftSight (Dassault) only 2D is free.
P.S.: Run a Google Search “.dwg viewer”. Even AutoCad has a free viewer.
There are open-source packages with limited DWG support, LinuxCAD comes to mind.
Some other software can be open-source (for the paying user) but not free (as in beer):
Short answer: No
Long answer: Yes, but…
The DWG-format is deep into licensing issues, theoretically you can set up your computer to be able to write/read DWG drawings (http://www.opendwg.org/), but you’d have to be a member and to pay a licensing fee. There are initiatives in the GNU-project (LibreDWG - GNU Project - Free Software Foundation), I’m not sure anything concrete has been achieved yet.
There’s a nice article outlining the issues at LibreGraphicsWorld. To quote the article:
“While file formats such as STEP (ISO
10303) exist, DWG is still part of far
too many production workflows.”