I just went looking for this again to install on my machine and I wanted to not only remind myself for the future, but also share it with others.
The Unarchiver is a replacement for the BOMArchiveHelper.app that comes with MacOS X. You drag it to your Applications folder, run it once to setup which files it handles, and then pretty much forget about it. From then on, it will handle opening most of the compressed file formats that you find on the Internet, including RAR, 7-zip and Stuffit files.
You can’t beat the price (free!) and once you’ve installed it, you’ll wonder why you stuck with using the built in support and a bunch of one-off utilities for each of the various other compressed file types.