If you’re using a slightly old version of AutoCAD, you’ll find yourself cursing those who send you drawings in the latest format because they won’t open properly on your system. Clean up your language – DwgGateway is here to help. It can open and save-as from and to just about any version of *.dwg files.

Lest you think this is the work of some frustrated guy who can’t afford the latest from AutoDesk, think again. DwgGateway comes to you from SolidWorks who probably feel that if you can’t beat the giants, you can at least be a thorn in their flesh.

Oh, and while this is a Must Have software, please don’t get excited about its PDF writing capabilities – they’re nothing to write home about.
http://www.dwggateway.com/

EDIT: This software seems no longer available from SolidWorks although there seems to be a similar download available here that claims to have been recently updated.

Alternatively, you might want to download DraftSight which is a free 2D CAD software for Windows, Mac and Linux and can save further back than AutoCAD itself.