Just want to view a DWG quickly? I have found nothing better than DWG Viewer from Brava. (But the free version is being killed.)
If you need editing and measuring, etc. DraftSight is free CAD software from Dassault. It requires registration, but it’s rather heavy-duty and the price is pretty good.
Autodesk has some Free DWG viewing software. Last time I tried their TrueView app, I found it to be just too big to be convenient for a quick viewer because it seems to be full AutoCAD with all of the editing tools turned off. It would surely be the most accurate, though, because it’s from the mother ship. Design Review is also said to work to view DWGs but this is what you see if TrueView is not installed:
Autodesk is also offering a web viewer, which uses their Forge backend.
It takes a moment to display but has sharing and markup features.