Abrir una terminal y ejecutar el comando adecuado a la versión de Ubuntu en la cual se esté por instalar Skype:
Skype For Ubuntu 13.04
sudo sh -c "echo 'deb http://archive.canonical.com/ubuntu/ raring partner' >> /etc/apt/sources.list.d/canonical_partner.list"
Skype For Ubuntu 12.10
sudo sh -c "echo 'deb http://archive.canonical.com/ubuntu/ quantal partner' >> /etc/apt/sources.list.d/canonical_partner.list"
Skype For Ubuntu 12.04
sudo sh -c "echo 'deb http://archive.canonical.com/ubuntu/ precise partner' >> /etc/apt/sources.list.d/canonical_partner.list"
Skype for Ubuntu 11.10
sudo sh -c "echo 'deb http://archive.canonical.com/ubuntu/ oneiric partner' >> /etc/apt/sources.list.d/canonical_partner.list"
Skype for Ubuntu 11.04
sudo sh -c "echo 'deb http://archive.canonical.com/ubuntu/ natty partner' >> /etc/apt/sources.list.d/canonical_partner.list"
Luego, actualizar:
sudo apt-get update
Por último, instalar:
sudo apt-get install skype
