Ubuntu Sources List Generator is a website where you can generate a sources.list file with a lot of useful repositories. Besides the usual Ubuntu main, restricted, universe, multiverse, updates and so on you can choose from many others. All you have to do is select your Ubuntu version on top of the page, and you will be able to select from the following repositories, under "3rd Parties Repos": Amarok 2, AWN, Banshee, 9+1 great Ubuntu complete themes (including gdm, emerald and awn themes), Cairo Dock, Chromium, Conky, Deluge, Gnome-Colors, Gnome DO, Google Linux Software Repositories, Medibuntu, OpenOffice.org, Mozilla Daily Build Team (Firefox 3.5), both Opera stable and Opera beta, Skype, VLC, Wine, XBMC Media Center and many others.
Debian users: there is also a Debian version of the Sources List Generator, so you may want to check it out.
Once you have selected all the repositories you want, click the "Generate" button and you will then be able to copy the repositories into your sources.list file:
For adding GPG keys of new repos, see instruction (if available) in the generated sources.list itself. To add the GPG keys for all the repositories at once, see this script which automates the key importing process.
Debian users: there is also a Debian version of the Sources List Generator, so you may want to check it out.
Once you have selected all the repositories you want, click the "Generate" button and you will then be able to copy the repositories into your sources.list file:
gksu gedit /etc/apt/sources.list
For adding GPG keys of new repos, see instruction (if available) in the generated sources.list itself. To add the GPG keys for all the repositories at once, see this script which automates the key importing process.