LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
FreeCAD - An open-source parametric 3D modeler
NttCAD - Free 2D/3D CAD software, easy to use and multi-platform
SketchUp - 3D for Everyone
QCAD - QCAD is an application for computer aided drafting in 2D.
KediCAD - CAD/CAM/ FEA software for Linux