LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
FreeCAD - An open-source parametric 3D modeler
gCAD3D - 3D - CADCAM - Software for MS-Windows and Linux. Freeware
SketchUp - 3D for Everyone
nanoCAD - nanoCAD is a pro-grade 2D CAD system with native DWG support and full customizability.
Autodesk AutoCAD - Autodesk AutoCAD is a commercial computer-aided design (CAD) and drafting software application.