• https://launchpad.net/~do-core/+archive/ppa (Gnome Do)
• https://launchpad.net/~transmissionbt/+archive/ppa (Transmission)
• https://launchpad.net/~pidgin-developers/+archive/ppa (Pidgin)
• https://launchpad.net/~awn-testing/+archive/ppa (AWN)
• https://launchpad.net/~gwibber-team/+archive/ppa (Gwibber)
• https://launchpad.net/~liferea/+archive/ppa (Liferea)
• https://launchpad.net/~webkit-team/+archive/ppa (Webkit) (Required by Liferea)
• https://launchpad.net/~chromium-daily/+archive/ppa (Chromium)
• http://ubuntuzilla.sourceforge.net/ (Ubuntuzilla - Firefox/Thunderbird Backports)
• http://www.virtualbox.org/wiki/Linux_Downloads (VirtualBox)
• https://launchpad.net/~dell-team/+archive/ppa (Dell)
• http://medibuntu.org (Medibuntu)

Adding PPA Key:

$ apt-key adv --keyserver keyserver.ubuntu.com --recv-keys $KEY

If the VirtualBox PPA throws an error after key import, do:

$ rm -f /var/lib/apt/lists/partial/*