Open source CAD software
LibreCAD is one open source project that I found promising. It's GUI is very similar to AutoCAD. This makes it quite easy for AutoCAD users. At present it does not support .dwg format. But you can work on dxf files.
If you're looking for free as in I don't want to pay money for a CAD program, I recommend you look into Draftsight
edit: Just in case, you wanted to learn the software or you're coming from AutoCAD, I've developed this tutorial / cheat sheet: http://97.107.136.148/Autocad_tutorial_v1.pdf
a few years ago. Unfortunately, since it's a numeric IP address, Stackexchange, doesn't allow it to be a link
I haven´t tried any of those yet, but these might be worth taking a look:
FreeCAD, BRL-CAD, Archimedes