The sources should have all the standard Debian packages you can use a utility like netselect-apt to automatically configure your soutrces.list file.
apt-get install netselect-apt
netselect-apt -n <distro> ( Insert Stable, Testing or Unstable in place of <distro depending on which you are running... )
If you are running Debian Stable (which is common for people that want a Very Stable system, also the most common install for new Debian users. ) you may want to look at debian-backports. Debian Stable is a EXTREMELY stable distro rock solid you might say, _but_ the packages in it are pretty out of date. Debian backports is just what you need to continue running Debian Stable but still get things like Firefox 1.5 or OpenOffice 2.0
http://backports.org/
This is my entire source list. I am running Debian 3.1 Unstable (also refered to as sid)
The 2 additional entries are for the repository that contains VLC Media Player and the Decss decoder so I can watch DVD's.. I have not found any need for additional repositories. the sources.list file doesn't need to be complex or large. there is more info on the sources.list in the Apt How-to I refered to earlier.
info on VLC repositories
http://www.videolan.org/vlc/download-debian.html
to add the debian-marillat repository for your specific ..
For sarge (stable) i386
Add in your /etc/apt/sources.list
deb ftp://ftp.nerim.net/debian-marillat/ sarge main
For etch (testing) i386
Add in your /etc/apt/sources.list
deb ftp://ftp.nerim.net/debian-marillat/ etch main
For sid (unstable) i386
Add in your /etc/apt/sources.list
deb ftp://ftp.nerim.net/debian-marillat/ sid main
another repository that may be of interest is the debian-unofficial.
http://www.debian-unofficial.org/index.html
it contains things like decss2, skype, Lame, Adobe Acrobat reader 7, etc...