AutoCAD is the most well-known computer aided design (CAD) product on the market. Whether you are working in 2D or 3D - designing mobile phones or bridges - AutoCAD has the features you need to make it perfect. AutoCAD modeling capabilities has helped many people through the years and together with it’s rendering features it has provided visualization of their designs. A powerful feature of AutoCAD is it’s scripting capability with which you can either extend functionality of AutoCAD - or perform automated tasks.

QCad Community Edition
Available for: Linux and Unix

Is an open source edition of RibbonSoft’s CAD system. QCad is a 2D CAD application that aims at being simple enough for anyone to use - even users with no previous CAD experience. Compared to the Professional edition the Community Edition has no scripting or poly-line support - also features are a few months behind. Features include layers, dxf files import and export, many flexible drawing tools and precise selection tools.

BRL-CAD
Available for: Windows, Mac, Linux and Unix

Is an open source cad systems based around constructive solid geometry (csg) modeling. On top of all of it’s modeling features, such as the interactive geometry editor, BRL-CAD offers parallel ray-tracing, path-tracing, image and signal processing tools. The ray-tracing process can even be distribution over a network of computers. When starting a new model determining an accurate unit of measurement ensures that your models will always have the correct proportions compared to real world objects. CSG is a fascinating way to work with 3d modeling and allows you to quickly build rough models and can be finalized later with all required details.

Archimedes
Available for: Windows, Mac, Linux

Is a computer aided design (CAD) solution focusing on architectural design. While the current version of Archimedes may not be mature enough for all - this project is very promising. It already has many of the most common and advance drawing features you will find in a commercial CAD solution. However, it’s interoperability with other products is still lacking. On the other hand drawing can be exported in scalable vector graphics (SVG) format - allowing you to share finished drawings with a wide audience without requiring them to install the product.
This is a great upcoming project started in Brazil - their goals are to create a real alternative for AutoCAD, but make it more suited for architects.