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StuffIt was to classic Mac OS what ZIP was to Windows. From the late 1980s through the Mac OS X transition, .sit was the standard archive format on Macintosh. StuffIt Expander shipped with every Mac, and downloading software from the internet meant downloading .sit files. The format supported Mac-specific features like resource forks that ZIP couldn't handle.
Aladdin Systems (later Smith Micro) developed StuffIt into a commercial product suite with StuffIt Deluxe, StuffIt Expander, and DropStuff. The .sitx format added better compression and encryption. But when Mac OS X arrived with built-in ZIP support and resource forks became obsolete, StuffIt's reason to exist evaporated.
Today, .sit files are encountered only in old Mac software archives and retro computing collections. The Unarchiver (free, Mac) extracts .sit files. On Windows, StuffIt Expander (if you can find it) or some versions of WinRAR handle them. For any new archiving needs, use ZIP.