The Unarchiver is a much more capable replacement for "BOMArchiveHelper.app", the built-in archive unpacker program in Mac OS X. The Unarchiver is designed to handle many more formats than BOMArchiveHelper, and to better fit in with the design of the Finder. It can also handle filenames in foreign character sets, created with non-English versions of other operating systems. I personally find it useful for opening Japanese archives, but it should handle many other languages just as well.
It is very simple to use and install - simply copy it into your Applications folder or whereever you prefer, and then set archive filetypes to open using it. This can either be done the usual way, or by double-clicking the icon to show The Unarchiver's preferences.
Supported file formats include Zip, Tar-GZip, Tar-BZip2, Rar, 7-zip, LhA, StuffIt and many other more or less obscure formats. Support for so many formats is achieved by using the libxad unarchiving library.
The Unarchiver requires Mac OS X 10.3.9. There is a Google Code page for The Unarchiver, where you can report and track bugs.