NanoCAD v5 uses a native .dwg format and an interface nearly identical to AutoCAD 2010. The free version is legal for commercial use (up to a point). While not "portable" out of the box, you can install it to a USB drive using a tool like or ThinApp to create your own legal portable version.
AutoCAD 2010 was a milestone release that introduced the .dwg 2010 file format, which is still widely compatible today. For many, the "Portable" version is considered better because it eliminates the bloat of modern installers. You get a streamlined experience that launches quickly from a USB drive without tethering yourself to a single workstation. Key Advantages of the 2010 Version portable autocad 2010 better