One of the leaders of the Django project has written a series of articles about how to write great documentation. I know pretending to know how to do that it is somewhat pretentious, but if you've read the Django's docs, you'll know that whatever he has to say about that subject is worth reading; Django is one of the best documented projects I know.

You can find the articles here. They are great!

Reading these made me remember of Producing Open Source Software, written by Karl Fogel, one of the authors of Subversion.