I like QCad, available from http://www.ribbonsoft.com/qcad.html>http://www.ribbonsoft.com/qcad.html. The 1.x versions are free and it works on DXF files which are the AutoCAD standard. It has Windows & Linux versions which is a big plus for me. Its 2D only and I expect a professional draughtsman would find it a bit lacking, but it does everything I need.