Computer-Aided Design Lovers Only:
DraftSight is a computer-aided design package similar to the pricey AutoCad software, and while there are a number of other free (I love free) CAD options, DraftSight can read and write .dwg and .dxf (i.e. AutoCad's) file formats. The software has to be registered periodically, a small inconvenience for "libre software".
Available on Windows, Mac, Ubuntu Linux and Fedora Linux.