The Unarchiver is designed to handle many more formats than BOMArchiveHelper, and to better fit in with the design of the Finder. It can also handle filenames in foreign character sets, created with non-English versions of other operating systems. I personally find it useful for opening Japanese archives, but it should handle many other languages just as well.
Once you have it in your Applications folder, just open double click on it and mark which type of files you’d like it to be in charge of opening.
Update: We don’t usually announce new versions, but this one is worth it. With version 2.7, The Unarchiver can now handle .sit files so you can finally get rid of Stuffit.
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