The most popular proprietary archive/compression format on the Mac. Developed & owned by Allume Systems [http://www.allume.com/] (previously Aladdin Systems).
Known for annoying and incompatible changes between major versions. If possible, use tar/gzip or compressed DMG on OSX.
Or, if you can live with 10.3 and later support, make a ZIP archive using the Finder’s built-in “Create Archive” menu item. These archives can even be unpacked with 10.0, 10.1 and 10.2 General/StuffIt versions, but you need to be careful (no resource forks, no names longer than 31 characters).
Allume provide an SDK for third parties to license General/StuffIt technology and embed it into their applications, FWIW.