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://www.debian-unofficial.org/ - Provides packages not available within the official Debian repository. It contains packages which are not distributable within Debian due to special license terms and packages which are not included in Debian due to political reasons (e.g. patent infrigement, binary-only/no sources, or special restrictive licenses).
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)
http://ftp.debian-ports.org/debian/ - Provides avr32, m68k, sh4 and sparc64 ports. There's several known ?DebianPorts/Mirrors.
http://www.debian-multimedia.org/ - Provides (mostly multimedia) packages for stable/testing/unstable which aren't or can't be in the official distribution (licences...).
https://eurynome.mirbsd.org/debs/ (Repository Index) – Provides adapted versions of packages (like mksh, openntpd, rdate) and new packages, including backports to etch/hardy/lenny as the need arises. Inofficial, but tries to produce good quality and standards compliant packages.