[update] As Carla Hufstedler reports in the comments, this seems to be gone now.
The Ubuntu sources.list generator, or Source-O-Matic, is a web application that can create custom sources.list files for various version of Ubuntu. The sources.list file tells the packaging system where it can look for software to install. The generator can add many additional sources, including those for proprietary software.
With this generator you can create your own custom sources.list from various available sources. It takes only two simple steps.
Remember to back up your existing sources.list file in case you have problems with the new one.