External repositories provide application or versions that are not in the official ones. You can find some very interesting stuff there. Most of them are not managed by Debian project. Here is a list of good places:
http://backports.org/ - Provides packages from Debian testing, recompiled (and sometimes adapted) to run on stable Debian releases.
http://apt-get.org/ - Package search engine, covers many inofficial repositories.
http://volatile.debian.net/ - Provides updates for Debian releases of packages with fast-moving targets - for example virus scanners and spam filters.
http://snapshot.debian.net/ - Provides historical Debian packages (all daily mirrors)
Some much renowned upstream developers (companies) provide repositories for their software that are not free in the sense of the DFSG (Debian Free Software Guidelines):
http://deb.opera.com - a well-perceived web browser
http://www.skype.com/download/skype/linux/repositories.html - IP telephony and messenger