RAR Expander is a MacOSX program which extracts the files contained in RAR archives. It supports both single and multi-part archives, and has support for password-protected archives as well. It uses the official unRAR library internally so it is fully compatible with archives produced by WinRAR.
RAR Expander also features AppleScript support, and includes a few useful example scripts for expanding multiple archives at once.
After first run it automatically registers as a default application to handle rar extension. Simple and handy.