Argh. Encouraging people to use Stuffit is nearly criminal. Bloated, packed with adware, greedy with filetypes - it's a menace. If you're unfortunate enough to be given a .sitx file you may be forced to use their fairly awful free Expander, but paying for the full version would surely be grounds for a psychological review. If you really need to compress things that tiny bit more than normal .zip, there's always 7zip.