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Second recommendation for the Unarchiver. It's a great little app, handles just about any archive format and is a whole lot less buggy than the abomination that is Stuffit Expander.

.sit is a dying format, frankly - it was only ever popular on the Mac, never making any real inroads anywhere else, and since OS X started to .zip utilities as standard has pretty much disappeared. Good riddance, I say :p
 
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