LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
SketchUp - 3D for Everyone
TigerCad - A completely free, simple technical drawing software (CAD)
A9CAD - 2-dimensional CAD program supports industry standard DWG/DXF drawing formats
FreeCAD - An open-source parametric 3D modeler
QCAD - QCAD is an application for computer aided drafting in 2D.