The Unarchiver is a small and easy to use program that can unarchive many different kinds of archive files. It will open common formats such as Zip, RAR (including v5), 7-zip, Tar, Gzip and Bzip2. It will also open many older formats, such as StuffIt, DiskDoubler, LZH, ARJ and ARC. It will even open other kinds of files, like ISO and BIN disc images, some Windows .EXE installers. The list is actually much longer - see the program homepage for the full list.
The Unarchiver also tries to detect and correctly handle the filename encoding in the archives it opens, allowing you to open files from every part of the world without getting garbled filenames.
The Unarchiver aims to be the only unarchiving program you will ever need, and to stay out of your way.
However, if you want to browse the contents of archives, or if you want to support development of The Unarchiver, look for "The Archive Browser" on the Mac App Store, or go to http://theunarchiver.com/archive-browser! It lets you browse and preview archive contents, and find out information about archives!
Also, if you need to open archives on your iPad or iPhone, look for "Archives" on the App Store, or go to http://theunarchiver.com/archives. It is based on The Unarchiver, and lets you easily handle Archives on iOS!