Medibuntu is probably the most useful and popular non-included repository, as it contains a lot of codecs for viewing or creating audio and video files. It’s a must have repository for Ubuntu.

• Acrobat Reader
• Firmware for the ALSA sound system
• Google Earth
• DVD decryption
• MPlayer / MEncoder
• Non-free codecs and
• Skype

and the list goes on. Two simple commands get you up and running:

sudo wget http://www.medibuntu.org/sources.list.d/jaunty.list –output-document=/etc/apt/sources.list.d/medibuntu.list
sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update

This adds the Medibuntu as a repository and updates the sources list. You can now install software like additional codecs, utilities and more like this:

sudo apt-get install non-free-codecs libdvdcss2

And you can always use Synaptic to install new software, of course. For a complete list, go here.