How do I open a CAB file on Mac OS?

The Unarchiver supports them too.

You may use the freeware StuffIt Expander 2010.
It supports over 30 archive file formats, including CAB.

The Unarchiver failed to open my particular CAB file ("error decrunching"), so I tried another pretty open-source app, called keka, which could do it.

There is also a cross-platform command-line tool that works: cabextract.