Drupal, on it's own, is a very powerful Content Management System. Jon Lebkowsky mentioned it before in The Real Value of the Dean Campaign here at WorldChanging.com already - but in this entry, I'll focus on it's actual use for collaborative documentation.
As a tool for collaborative work, Drupal is exceptional - though there are things that need to be understood, such as it's taxonomy system - which is very much like a hierarchy, only content can belong to multiple branches of the same hierarchy - at different levels. This is quite powerful, as it can make the same information easier to find for different people, and also allows different subjects to use the same material.
For the eAsylum remix of Free Culture, we used the Book module, which is extremely powerful - and something which we took to the limits (and which caused me to gain a few more grey hairs last weekend). Yet it worked; the problem we did have was due to the size of the text we were manipulating (we crammed entire chapters into book pages!) and the revisioning system of Drupal. I singlehandedly broke the Drupal module by using the revisioning; I also singlehandedly fixed it. For large amounts of text with lots of revisions, I would suggest using smaller 'pages'.
The image above is actually a capture of part of the administration of the book itself. The 'weight' allows one to move pages within the book; the lower the weight (negative numbers being smaller), the higher it appears in the book. This is quite useful, and allows spontaneous 'mixing' of the book. You'll also note that the subsections were easily handled in a similar fashion ('Piracy' and 'Property' in the image), which allows even more control over the content. When collaborating over documentation for any project, sometimes where it is and how it appears is very dynamic.
Without Drupal, the Remix of Lessig's book, Free Culture, would have been very difficult. Other tools such as Wikis would have made the task almost as simple, and arguably simpler, but this particular project showed that Drupal was up to the task.
With built in RSS feeds, built in Forums, and the ability to handle weblogs for multiple users, it's simply amazing what Drupal can do. And since it's licensed under the GNU General Public License, adaptability is one of it's strongest points. It's cost, too, is a big selling point - it's available at no cost.
Do you have to be an expert to use Drupal? Not really. You do, however, require a vision and the ability to learn - because with all the features Drupal packs in, it's almost impossible to document every possibility. And if you run into problems, there are always the Drupal Forums.
For an even better look at what Drupal can do, take a look at the Drupal site. Drupal 'eats it's own dog food'.
Because of my experiences with Drupal, I expect that in the near future I'll be developing my own modules for projects that I hope are worth mentioning here.