There are new Autodesk products (AutoCAD LT or similar) for Mac OS and iOS, but I couldn't find a simple free DWG viewer for Mac.
5 Answers
A360 Viewer
AutoCAD's A360 Viewer is a browser based tool for viewing DWG
files:
A360 Viewer Free online viewer - View designs in more than 50 file formats, including .dwg, right in your browser—no downloads or plug-ins needed.
Edit: 2021-11-26: A360 viewer requires Autodesk subscription to be able to upload and view .dwg files.
LibreCAD
LibreCAD is open source, free, and can read and write DWG
files.

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AutoCAD WS is a free app on the Mac App Store. I've never tried it.

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1Has 16MB file size limit. And you have to register to Autodesk (or already have their account). Otherwise seemed okay. – akauppi Mar 18 '13 at 14:28
The free eDrawings Viewer for Mac claims to be able to read and display DWG files. The Microspot DWG Viewer also provides support for DWG files.
I don't have any DWG files to test these two products on at the moment, but they're worth a shot.

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I did try the eDrawings Viewer for Mac but it doesn't allow do measurements and printing is not great neither. I finally found viewer which has all what I was looking for - ZWCAD Viewer. Get it from "cnet.com" as I couldn't find it on the ZWSOFT home page.

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I've found "Fast Cad Reader" on Mac App Store. It is incredibly fast and easy to use. It is only for simple DWG file reading, does not have pro features but I think it is far by the best -just view- app available for mac.
https://apps.apple.com/tr/app/fast-cad-reader/id1484905765?mt=12

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