I recently bought a new Macbook and discovered that it did not come with stuffit installed. When I realized that I probably now had some choice as to what to use, I thought I would explore other options for various reasons. What else is out there that can open .sit's?
The SIT format can really only be opened with StuffIt or the free StuffIt Expander. Thankfully, Mac users are slowly ditching the format in favour of ZIP or DMG, both of which are fully supported in Mac OS X without any extra software required. I also use RarOSX as a way of managing very large archives - it has a few advantages that make it quite useful.
For opening .sitx files, I don't think there are alternatives to StuffIt on Mac OS X. Like others on these boards, I recommend that you move from sitx as it is not as widely supported as something like Zip or BZip-2.