rtorrent on GNU/Linux
Installation on ubuntu dapper (the source part is probably working on other distros to)

1. From source; suitable if you want the newest version or need a fix for a annoying bug.
Do the following steps in a terminal.
First upgrade your system by doing:
sudo apt-get update && sudo apg-get upgrade (press enter)
Now lets download libtorrent and rtorrent, libtorrent is a dependencay of rtorrent so we need it.
wget http://libtorrent.rakshasa.no/downloads/libtorrent-0.10.1.tar.gz (press enter)
wget http://libtorrent.rakshasa.no/downloads/rtorrent-0.6.1.tar.gz (press enter)
tar xzfv libtorrent-0.10.1.tar.gz (press enter)
tar xzfv rtorrent-0.6.1.tar.gz (press enter)
Now we got the source and we have unpacked it.
Ubuntu does not come with gcc and some packages needed for rtorrent to compile, do:
sudo apt-get install build-essentials (press enter)
Now we need some build-time dependencies:
sudo apt-get install libsigc++-2.0-dev libncurses5-dev (press enter)
Now we got everything needed for compiling rtorrent, do:
cd libtorrent-0.10.1/
./configure
make
sudo make install
cd ..
cd rtorrent-0.6.1/
./configure
make
sudo make install

Now rtorrent should be in installed and in a usable state.

2. From apt-get easy and fast install
sudo apt-get update && sudo apg-get upgrade && sudo apt-get install rtorrent