CADuntu is a 2D CAD drawing tool based on the community edition of QCad ported to Qt4 and works natively on OSX, Windows and Linux.
Bricscad V10 for Linux was released earlier this month (the stable version) and is the first .dwg based CAD software available for Linux - according to its website, although VariCAD also has .dwg support (I'm not sure about the difference between "support" and "based" in this case though).
We recently posted about a free AutoCAD alternative called BRL-CAD, which is very advanced and build by the US military in collaboration with NASA. But the application is not that easy to use by normal people, not very much into CAD applications (or the army :-) ).
AutoCAD is one of the most expensive applications, but also indispensable for many types of jobs. Being so expensive, it's a good reason to look for free alternatives. QCad is one of the alternatives but sometimes it's just not enough.