AXCAD

Axcad is a 100% DWG compatible and AutoCAD command compatible CAD is another affordable choice for AutoCAD users.

100% DWG compatible, using DWG as native file format. You won't lose any dwg files to use this affordable CAD. Supports AutoCAD 2006 dwg format (from r2.6~2006)
High degree of AutoCAD command set compatible. You won't spend much time on learning AXCAD.

Supports LISP and ADS development.