At a time when I was using Red Hat (Fedora), One of my favourite repositories was Dag-wieers not only because the official Red Hat repository was dead slow due to excess traffic but also because dag-wieers contained a number of additional RPM packages which were missing in the official repositories such as those with support for proprietary file formats. That was the culmination of my search for additional repositories to include in my Yum configuration file.
Now a days, this is not at all a problem especially when you are using Ubuntu, as the repositories have been demarcated into different sections such as Universe, Multiverse and so on depending upon the type of package available in each one of them such as whether the package is released under a free license or a proprietary one. And it is only a matter of enabling the desired repository and then using apt-get to install the requisite package. Still, it doesn't hurt to have a number of additional repositories apart from the ones provided officially by Ubuntu. Trevino has compiled an exhaustive collection of repositories for Ubuntu and Kubuntu which you can include in your /etc/apt/sources.list file. A word of caution is in order though, which is that since these are unofficial repositories, it is difficult to verify the integrity of the packages. So use at your own risk.
