Two months ago, a case study about the migration of France 24 from Drupal 5 to Drupal 6 was posted on drupal.org frontpage, and it was announced some really interesting modules were going to be open sourced.
I'm happy to say it is (almost?) finished, and here is the list of modules:
Besides the Handbook on drupal.org, there is also some Drupal documentation that is embedded in the Drupal source code, and visible from the administration pages of a Drupal site. This includes help pages for modules, help text displayed at the top of administration pages, and descriptive text within administration pages (e.g. help text for a setting field).