I ask because I am in need of some examples of spectacular documentation to do a little compare/contrast. Bad documentation abounds but who gets it stone cold correct?
I suppose we should differentiate between documentation geared at informing experts and documentation geared at teaching new users but if you know of something that combines both of these, by all means reveal!
I've found the Django documentation to be quite useful. Its tone is conversational, with snippets interesting for beginner visually distinguished from the main reference text, so as to be easily filtered out by the expert user. Its style is visually clean and textually logical, in that it closely tracks my thought process from "WTF is Django" do "how do I solve specific Django implementation issue X for my project?"
PHP - far and away the most useful documentation i've ever seen.
It makes it very easy to find out new functionality of the language and very quickly understand how to use it, when to use it, when NOT to use it and what you should perhaps use instead. The user-contributed notes on each page often give insights into other interesting ways to use the function.
MSDN definitely is the best documentation out there. They have most of their technologies documented and most of the times they show you examples and code snippets and not just the reference. I believe that is one of microsoft strengths.
Django also has good documentation and quite simple. Very Ad-hoc to the framework.
The Commodore-written documentation for the Amiga OS was superb. I learned probably 80% of what I apply today from those books and text files/AmigaGuides. The RKM Libraries Third Edition had probably the most cogent explanation of OO programming that I've written - understandable even to a 14-year old as (I think) I was then. Ah, happy days.
Link to online versions: http://pub.elowar.com/AmigaDev/
PostgreSQL has an excellent documentation, especially for all it's SQL syntax.
Does paper documentation count?
Apple's old "Inside Macintosh" series is, by and large, the best set of written docs I've ever read.
Yes, I have the white hair to prove that I actually did read it :)
What kind of documentation are you looking for? If you're talking about API documentation, Java makes it easy to generate really good documentation via javadoc. People generally fail to take advantage of it though.
Anyway, I usually look at the following when trying to figure out appropriate conventions (grammar style, levels of details etc):
Some of the best documentation I've ever seen is in the CodeIgniter project. The whole API is outlined with examples along the way, and the user interface is incredible. They really took their time making it presentable.
Not to mention the Wiki and the forums. With those added in, what else could you ask for?
I learned SQL from the MySQL documentation. I'd say it's the best documentation I've ever read.
So many shares the view that The TexBook by Donald Knuth is the best software documentation ever.