The editor emacs continues to amaze me. For some time now I have been using emacs as a day-planner in the excellent org-mode. Once you get used to the commands it's a breeze to create documents with structured headlines, internal and external links, etc. It's also very versatile in that it can export the same document to different formats like html or ascii. The stable version that comes with Gentoo unfortunately does not seem to support docbook format (latest upstream stable version does however and possible to use with Gentoo if you run the keyworded version at the time of this writing).
You can find a nice screencast here. Org-mode was also featured with Google Tech Talk.