I use expander for a long time. Though I personally don't use sit-files any more (they do not bring any advantages any more as far as I know but are less "compatible" than other files), I'm very happy that there is Expander, now working flawlessly for me on Leopard. It decompresses not even the .sit files I sometimes still get but simply anything I ever came across, let's see, the official list is this: .sitx, .sit, .zip, .sea, .tar, .bin, .hqx, .cbr, .bz2, .tgz, .lha, .mime, .rar, .arc, .gz, .cbz, .uue, .exe, .cab, .yenc ,.z ,.pf, .uu.
Have you noticed? Not only all the superb compressions of the unix world like .gz, .bz, .rar and so on but even self-extracting .exe files work (on a Mac!) with this utility. I do not know if there is a decompression utility out there as simple and powerful - I don't know one. And in fact I don't need any other, for Expander alone is enough.

PS: Just to avoid misunderstandings: 1) Stuffit Expander is a simple Standalone App, that does not interfere with your system like somebody claimed. - Please do not confuse it with Stuffit Standard or Stuffit Deluxe. 2) It's free and you do not even have to give your email address (although smithmicro tries to get it), please check the post of amcgee below for the link.