Some software projects and some Debian developers provides extra repositories that you can use besides the official ones. There are various reasons why the software availaible in these repositories is not packaged in Debian. It could be because the software is not yet in Debian (dotdeb), legal concerns (deb-multimedia), or because the software itself is closed-source (Oracle,Skype,Opera). We use the following tags to describe the repositories, to get an idea of the trust you can put in these repositories.
DD : The repository is managed by a Debian developer or a Debian maintainer
FLOSS: the repository contains only free/opensource software
COM: the repository is backed by an external commercial identity.
These tags can be combined
NB: The apt-get.org package search engine, covers many inofficial repositories, but is now outdated.
Provides adapted versions of packages (like mksh, openntpd, rdate) and new packages, including backports to etch/hardy/lenny as the need arises. Unofficial, but tries to produce good quality and standards compliant packages.