MediBuntu

Medibuntu (Multimedia, Entertainment & Distractions In Ubuntu) is a repository of packages that cannot be included into the Ubuntu distribution for legal reasons.

Its main focus is on multimedia support, something similar to the Combined Community Codec Pack for Windows.

The most interesting feature is a full version of ffmpeg (including MP3 and x264 encoding).

More help on adding the repository and installing individual packages can be found in the Ubuntu online help.