Another vote for UnrarX & The Unarchiver:
UnrarX is a very basic app an i mean that in a good way in that it won't clutter your system. The Unarchiver is more integrated into OS X, if you are only needing to deal with RAR files occasionally then UnrarX is the best solution, i use this app myself. If you are dealing with a high volume of RAR and or other compressed files then The Unarchiver is a better choice.