Hieraki
Using an idea that is obvious only in retrospect , Hieraki is a cool new application that does a simple job well. Maybe using Rails has something to do with this.
Take a wiki, and constrain its structure into a hierarchy, and you have a perfect solution for maintaining technical documentation. You get the benefit of distributed editing, without having to strain to retain a semblance of structure to guide your reader. Highly recommended. Its a niche that is far from crowded, even though there are thousands of wiki implementations out there. While many of the proprietary document management solutions are moving in this direction, Hieraki is the only open source app that I know of in the ‘use a wiki to write a book’ space.
In true open source fashion, of course, some goodness has started to flow from external contributions – I’ve been motivated to submit a patch for creating XSL:FO: (from which you can generate PDF’s) here, while a system for fine grained access control has been submitted here.