CAD Tools are essential for engineers and AutoCAD is a popular commercial CAD software. Though it is very powerful, it costs a lot of money. If you want a light weight CAD tools/software then try the following;
Innovate, design and collaborate at no cost. nanoCAD is a 2D/3D CAD for professional design and drafting. A combination of familiar interface, powerful tools, native *.dwg format at no cost makes nanoCAD a best solution to all your design needs.
FreeCAD (Open Source)
FreeCAD is a general purpose Open Source 3D CAD/MCAD modeler, aimed directly at mechanical engineering and product design but also fits a wider range of uses in engineering, such as architecture or other engineering specialties. It is a feature-based parametric modeler with a modular software architecture which makes it easy to provide additional functionality without modifying the core system.
DraftSight (Free for individuals)
DraftSight lets professional CAD users, students and educators create, edit and view DWG files. DraftSight runs on Windows, Mac and Linux.
DoubleCAD XT (Free)
DoubleCAD XT is an AutoCAD LT® work-alike. But free.
BRL CAD (Open Source)
BRL-CAD is a powerful cross-platform open source solid modeling system that includes interactive geometry editing, high-performance ray-tracing for rendering and geometric analysis, image and signal-processing tools, a system performance analysis benchmark suite, libraries for robust geometric representation, with more than 20 years of active development.
QCAD (Open Source)
QCAD is an application for computer aided drafting in two dimensions (2d). With QCAD you can create technical drawings such as plans for buildings, interiors, mechanical parts or schemas and diagrams. QCAD works on Windows, Mac OS X and many Linux and Unix Systems. The source code of the QCAD community edition is released under the GPL (Open Source).
OpenSCAD (Open Source)
OpenSCAD is a software for creating solid 3D CAD objects. It is free software and available for Linux/UNIX, MS Windows and Mac OS X.
gCAD3D (Open Source)
gCAD3d 3D – CADCAM – software for MS-Windows and Linux.