DraftSight

DraftSight is a professional, 2D CAD product, open for all users who want to draw, edit, and view DWG file.

DraftSight is free product of Dassault Systems which also have many commercial CAD products, like SolidWorks

DraftSight can be downloaded from Dassault Systems' official website or from   CNet

Scilab

Scilab is an open source mathematical tool for science and engineering, similar to Matlab. Unlike Matlab, Scilab is a free. You don't need to buy it. Scilab is compatible with three mostly used Operating Systems, Windows, Mac OS, and Linux/Unix. Scilab can be downloaded from www.scilab.org.