Fileinfo.net Is a site that collects file extensions, categorizes them by application type, and which programs both Windows and Mac can open.
You can do direct searches, such as .dwg, or also by file type, for example "CAD".
This is the result you get when selecting the files that have the word CAD in their description.
Very useful resource to know for example that Mac and Windows applications can open a .dgn file
Bentley View (free)
Bentley Systems MicroStation (commercial)
Cimmetry Systems AutoVue
Autodesk AutoCAD LDT