As of Karmic Koala Ubuntu introduces a new and simplified way to add repositories. From the command line you would just add
sudo add-apt-repository ppa:xorg-edgers/ppa
to register for example the Xorg-Edgers repository with APT and acquire the public key if required in one step.
I usually make use of a couple of third party repositories like Medibuntu, Xorg-Edgers, SMPlayer and XBMC SVN. Beeing a heavy user of Guake terminal this is just a very convenient and fast way to work with repositories.