If you just received a package with .rar extension from your friend and you cannot open it,this quick tip may help you.

We used to right-click on a file and select to pack/unpack it,however for rar we need to run following command first.
Open up a terminal window from Applications->Accessories menu and execute this command:

sudo apt-get install rar unrar
sudo ln -fs /usr/bin/rar /usr/bin/unrar

In addition,there’s a free software called peazip that supports handling many archive and compression formats.

PeaZip free archiver utility is a portable, open source file and archive manager.
PeaZip is cross platform and runs on 32 and 64 bit Windows (9x, 2000, XP, Vista) and Linux. It supports handling many archive and compression formats:
* create: 7Z, ARC, BZ2, GZ, PAQ/LPAQ, PEA, QUAD, TAR, UPX, ZIP;
* open: ACE, ARJ, CAB, DEB, ISO, LHA, RAR, RPM and more archive types…

Download the deb package and double-click to install:http://sourceforge.net/projects/peazip/
Peazip can be launched from Applications->System Tools menu (try restart X-server first)