Being able to decompress the plethora of archive types that are flying about the intertubes these days is essential. Stuffit Expander, the free extract-only part of the Stuffit suite is a great utility to have when Finder, or even the Unarchiver, just can't help you out.
There are quite a few free extraction tools available for the Mac. We've got the Unarchiver and TinyExpander just to name a few, but Smith Micro's Stuffit Expander handles several file formats that most others just can't, including Smith Micro's .sitx files, with aplomb.
It's simple to use, either open the archive from Finder with Stuffit Expander, drag-and-drop the file onto the Expander program or open the archive directly within the app. You can even just drag-and-drop your file onto Stuffit Expander's dock icon to quickly extract the file. If you're going to use it as your primary archive extractor you can also assign various different archive file formats to Stuffit Expander from within the program preferences.
When other extractors just can't cope with that unusual archive type, Stuffit Expander steps in. From MIME, StuffitX and yEncode, to AppleSingle, ARC and .btoa files, Stuffit Expander can get you what you need and for that reason, regardless of whether you use the Unarchiver or its kin, it's an essential free Mac app.
Stuffit Expander is available for free from the Mac App Store.