Hi,
I have on my Mac:
1. AutoCAD WS - free in Mac App Store - https://itunes.apple.com/us/app/autocad-ws/id454730552?mt=12
Requires Internet connection and Autodesk account to view DWG files, because it uploads them to the cloud before viewing. After uploading you can do a lot of things - "basic, intuitive set of viewing, editing, and markup tools to work on your designs...Plot your DWG files as a PDF or DWFx with full control over plot styles, paper sizes, layouts..."
Can open DWG format up to 2013
2. progeCAD Viewer DWG - free in Mac App Store - https://itunes.apple.com/us/app/progecad-viewer-dwg/id468632219?mt=12
Work absolutely "off-line", fast enough, no markup tools, only:
- full support for Layers (hide/show through Layers palette)
- compatibility with Model and layout visualization
- visualization of 3D Wireframe and Shade controlled via palette
- DWG export in DXF
- support of AutoCAD 2010/2012 DWG files with no file-size limit
Crashes sometimes...
You can also try free DraftSight application (not only viewer )
Maxim