AutoCAD drawing
DWG is the native file of AutoCAD, the program most architects and engineers draw in. It stores lines, arcs, text, dimensions, layers, blocks and the paper layouts of a drawing, and the format has changed with almost every AutoCAD release, which is why a file from a newer version often will not open in an older program. Most people who are sent a DWG only want to look at it or print it.
Opens in AutoCAD, and with import support most other CAD programs, DWG viewers and some free tools
DWG converter