The Unarchiver is a much more capable replacement for "Archive Utility.app", the built-in archive extraction program on Mac OS X. The Unarchiver is designed to handle many more formats than Archive Utility, 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 else you might prefer, and then set it as the default program for archive files. This can be done either from the preferences panel in the program itself, or in the Finder.