AddingRepositoriesHowto, LocalAptGetRepositories – Ubuntu Wiki (sometimes you need not only one package, but you have to provide a dependency like a library or whatever. When those dependencies are not in the official repositories, the only way to fulfill those dependencies is to put your package inside the repository.), AddingMultimediaRepositories – Ubuntu Wiki, How to Use Ubuntu Backports Correctly, BreakMyUbuntu (modeled onBreakMyGentoo) & PackageManagementHowto.
be careful, the repositories listed at apt-get.org are at the absolute bottom of the support ladder, and can include illegal or even malicious packages, and sometimes tend to fail even in their native Debian environment. [x]