The Medibuntu repository (Multimedia, Entertainment & Distractions In Ubuntu) contains packages that cannot be included into the Ubuntu distribution for legal reasons. Some of these packages include Skype Acrobat Reader, Google Earth, and multimedia codecs.
Add Medibuntu to your sources.list. Make sure you specify the correct version of your distribution. This example uses Intrepid Ibex (Ubuntu 8.10).
# wget http://www.medibuntu.org/sources.list.d/intrepid.list --output-document=/etc/apt/sources.list.d/medibuntu.list
Then add its GPG key to your keyring.
# apt-get update && apt-get install medibuntu-keyring && apt-get update
Now you should be able to install packages in this repository.