SuperNZB is a free download that includes a free RAR file decoder.
Download for Windows 7, Vista and XP
Download for Mac OS X
Once you have SuperNZB installed, just click the “RAR” button on the main window.
Here is a 3 minute demo video. It was made on Windows, however SuperNZB looks identical on the Mac:
We provide this free RAR decoder in order to promote SuperNZB.