LeoCAD
A CAD application for creating virtual LEGO models
...At the same time, it has a rich feature set that enables experienced users to build models by using techniques that are more advanced. LeoCAD is fully compatible with the LDraw Standard and related tools, and it reads and writes LDR and MPD files so that you can share and download models from the Internet. It also uses the LDraw parts library, which has over 10,000 different parts and continues to receive updates. Native versions are available for Windows, Linux, and macOS to ensure that users are familiar with the program interface. LeoCAD is open-source, so anyone can contribute with fixes and features, and it is always going to remain free.