Intellicad was developed by Shapeware (maker of Visio) and the assets were transferred to the Intellicad Consortium
www.intellicad.org before the assets of the company were sold to Microsoft. The consortium members jointly develop and share the source code and many use it to build products such as architectural design packages using the engine. I've used Bricscad and a thing called DWGeditor which comes from member Cadopia that is bundled with Solidworks. Various members sell for different prices so shop around if you need a less expensive Autocad clone.