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I'd actually highly suggest The Unarchiver over stuffit.

http://wakaba.c3.cx/s/apps/unarchiver.html

It's free and supports a wider variety of formats than stuffit (including those crazy cpgz), although it sometimes has problems with passworded RARs... but since that's not your problem, don't worry.
 
I agree this is a necro thread. Double clicking to unzip a file depends on the OS version. A newer OS will unzip a zipped file with a double click.
 
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