The very nice RAR Expander application icon that's also featured at the top of this page was designed by Amir Alam of "Druckerei Alam". Thanks for helping out Amir!
Other RAR Software
I started the Rar Expander project because I could not find a GUI-based program for expanding Rar archives on MacOSX that could support multi-volume archives. A more thorough search later revealed there were such projects after all. I'm not claiming that Rar Expander is the best, fastest or most user-friendly of these projects so I'm providing links to them here so you can make your own evaluation of which rar expansion program is the best for you (these links and descriptions of the projects may be outdated and incomplete):
From RarLab you can download the official commandline program for extracting RAR archives on MacOSX.
Aladdin Systems offers Stuffit Expander, a GUI-based program for MacOSX. It only supports single volume RAR archives, not multi-volume ones.
unRarX is a GUI which is a front-end for the unRAR commandline utility.
Unrar X is another GUI which works as a front-end to the unRAR commandline utility.
MacPAR deLuxe GUI: yes, multi-volume: yes.