I tried many sites and all are asking to sign-up to view a .dwg file.
The best online viewer is AutoDesk’s DXF/DWG Viewer
Yeah, you have to create an AutoDesk account but it’s free and it’s because you’re uploading your files. If you didn’t have an account you wouldn’t have access to them.
Also you can download True viewer it is free software from AutoDesk.
If you want to be able to view DWG files even offline, there are plenty of free CAD software you can download.
My favorites would be: