I am not a professional drafter, but for 15 years have been using a CAD program once a month for day or so. Here are my observations on what is available for an occasional user like myself. Please drop in an email with your reviews.
QCAD (GPL): http://www.qcad.org. This is a simple, easy-to-use, and overall surprisingly lovely and smooth program for 2D drawings. Both Linux and MS Windows versions of qcad are available. And best of all, it is completely free. I find it useful for simple drawing or learning. It uses the popular *.dxf file format as a default. However, for some reason, not everything exported from AutoCad as *.dxf is at the right location on the drawing when read by qcad (major pain for me).
VariCAD (proprietary commercial, a free 15-day trial version available for download, geared towards mechanical design, both Linux and MS Windows versions are available): http://www.varicad.com/. VariCAD is so-so as for the price but it is still way cheaper and offers fewer "product activation" roadblocks than "AutoCad Lite" while being more capable. I did not encountered any problems reading or writing *.dxf and *.dwg files. The menus appear disorganized to me, but it took me only 0.5 day of using varicad to feel comfortable with it (I have an AutoCad background). Their email support was immediate and helpful, quite contrary to what I became used to when dealing with Autodesk (the maker of AutoCad). I have no association with VariCAD whatsoever. Varicad is my current choice CAD program under both Linux and MS Windows.
OCTREE (free for non-commercial applications): http://www.octree.de/html/frames/eng/f_octree.htm
VARKON (LGPL): http://www.varkon.com/
Microstation (proprietary): http://www.microstation.com/academic/products/linux.htm--the academic edition of Microstation includes the Linux version of their excellent CAD system (better than AutoCad).
There is also something called "LinuxCAD" but it appears to be a rip-off (I really do NOT recommend it).[an error occurred while processing this directive]