Depends on what you want/need. If there is something you want that you can't find in an 'official' repository just search at
www.apt-get.org
#Stable Sarge
deb
http://http.us.debian.org/debian/ stable main contrib non-free
deb-src
http://http.us.debian.org/debian/ stable main contrib non-free
#Security Updates Stable
deb
http://security.debian.org/ stable/updates main contrib non-free
#Debian Unofficial packages for Sarge/stable
deb
http://ftp.debian-unofficial.org/debian/ sarge main contrib non-free restricted
deb-src
http://ftp.debian-unofficial.org/debian/ sarge main contrib non-free restricted
# Testing Etch
deb
http://http.us.debian.org/debian/ testing main contrib non-free
# Testing Sources
deb-src
http://http.us.debian.org/debian/ testing main contrib non-free
# Security Updates Testing
deb
http://security.debian.org/ testing/updates main contrib non-free
# Unstable Sid
deb
http://http.us.debian.org/debian/ unstable main contrib non-free
# Unstable Sources
deb-src
http://http.us.debian.org/debian/ unstable main contrib non-free
# Debian experimental
# deb
ftp://ftp.debian.org/debian/ ../project/experimental main
# deb
http://http.us.debian.org/debian/ ../project/experimental main contrib non-free
#various packages
#Debian KDE 3.4 Team updates
# deb
http://pkg-kde.alioth.debian.org/kde-3.4.0/ ./
# deb
http://pkg-kde.alioth.debian.org/kde-3.4.1/ ./
# Java
# deb
ftp://ftp.tux.org/java/debian/ unstable non-free
# deb
ftp://sunsite.dk/mirrors/java-linux/debian/ testing non-free
# deb
http://www.linex.org/sources/linex/debian/ sarge linex
#wine
deb
http://wine.sourceforge.net/apt/ binary/
deb-src
http://wine.sourceforge.net/apt/ source/
#mplayer,acroread.realplayer,various packages
deb
ftp://ftp.nerim.net/debian-marillat/ sid main
deb-src
ftp://ftp.nerim.net/debian-marillat/ sid main
# Star Control II - The Ur Quan Masters
deb
http://uqm.debian.net/ unstable/
#OpenOffice.org 2.0beta
deb
http://debian.tu-bs.de/project/kanotix/ooo-kanotix/ ./