If anyone happens to read this who still think using .sitx archives to distribute things over the internet is a good idea, please don’t.
OS X no longer ships with Stuffit Expander and downloading the free version requires you to give Smithmicro your email address to receive their download link.
Luckily most knowledgeable Mac developers don’t use Stuffit any more, but occasionally you’ll come across the odd printer driver or Icon set that presumably someone who mainly uses Windows has thoughtfully archived in a “Mac friendly” format. The hapless user will then install Stuffit, which of course promptly hijacks all your file associations.
For those of you who’ll argue that Stuffit can decompress more file types than the built-in “BOMArchiveHelper.app”, I recommend taking a look at “The Unarchiver”. Not only can it take care of those pesky .sitx files, it can also decompress .zip, .rar .7-zip and other obscure file formats.