LeoCAD - Virtual LEGO CAD Software
CAD application for designing virtual LEGO® models
QCAD
QCAD is a free, open source 2D CAD system for Windows, Linux and Mac.
BRL-CAD
Open-source solid modeling
LibreCAD - Free Open Source 2D CAD
FreeCAD
FreeCAD, the open source 3D parametric modeler