To extract rar archives in Ubuntu do this:
Open a terminal and past this command in it
sudo apt-get install unrar
Or if you are using Firefox click here.
This won't install another software on your PC, rather Ubuntu's Achieve manager will get the ability to work with RAR files. Unrar is a small 99 KB package.
Note that unrar isn't an opensource project, so those who live by opensource ideology won't like to install this.
Tombuntu blog has posted another opensource solution here. However the opensource unrar doesn't work with all rar files.