LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
FreeCAD - An open-source parametric 3D modeler
QCAD - QCAD is an application for computer aided drafting in 2D.
OpenSCAD - OpenSCAD is a software for creating solid 3D CAD objects.
NttCAD - Free 2D/3D CAD software, easy to use and multi-platform
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite