Gnus is the advanced, self-documenting, customizable, extensible unreal-time newsreader for GNU Emacs.
Oops. That sounds oddly familiar, so let's start over again to avoid being accused of plagiarism:
Gnus is a message-reading laboratory. It will let you look at just about anything as if it were a newsgroup. You can read mail with it, you can browse directories with it, you can ftp with it---you can even read news with it!
Gnus tries to empower people who read news the same way Emacs empowers people who edit text. Gnus sets no limits to what the user should be allowed to do. Users are encouraged to extend Gnus to make it behave like they want it to behave. A program should not control people; people should be empowered to do what they want by using (or abusing) the program.