Adding a software repository from Ubuntu PPA in Ubuntu Karmic and later

This new method is easier than the old one which required you to edit your software sources manually, and then get the key and import it. The new one is as follows:

1. Go to the PPA page of your interest, e.g. URL, check the name of the new repository, it is "ppa:transmissionbt/ppa" in my example.
2. Open a terminal and type "sudo add-apt-repository ppa:transmissionbt/ppa"
3. Done! This both adds the entry to your software sources list as well as importing the key which authenticates that repository.
4. Don't forget to refresh your repository to be able to update/install new packages "sudo apt-get update"

PPA = Personal Package Archives, i.e. the user-contributed software repositories.