A basic version of "unrar" can be installed simply with:
apt-get install unrar-free
However this is unable to handle the RAR v3.0 format (which most .rar files probably use). For this we must use "unrar-nonfree", which isn't in Raspbian's repositories. Not to worry, it can be installed easily enough using the APT system...
Firstly ensure that the following line exists in your /etc/apt/sources.list (and isn't commented - i.e. the line doesn't start with a #):
deb-src http://mirrordirector.raspbian.org/raspbian/ wheezy main contrib non-free rpi
Now it's a simple matter to install the application...
apt-get build-dep unrar-nonfree
apt-get source -b unrar-nonfree
dpkg -i unrar_*_armhf.deb