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On the iPhone? Well, PDF Expert can open RAR, for instance.

Edit: whoops, thought I was in the iOS forum. On my Mac I usually use Archiver for RAR files. But there are lots of RAR expanders for Mac.
 
I use The Unarchiver. Its free on the App Store and will handle most archive formats.
 
I use Unarchiver as well. It simply works without having to configure a lot of stuff, and it's noticeably faster than Windows apps like WinRAR that do the exact same thing.
 
I have three that I use--two mentioned here--unarchiver and MacPAR Deluxe (which can do additional things--especially handy if you like Usenet). One of the best still works in Mavericks although it is no longer supported--and it is still available to download--SimplyRAR.
 
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