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
gCAD3D - 3D - CADCAM - Software for MS-Windows and Linux. Freeware
QCAD - QCAD is an application for computer aided drafting in 2D.
nanoCAD - nanoCAD is a pro-grade 2D CAD system with native DWG support and full customizability.