I recently started using DocBook XML to author my documentation.
On the upside, it's a pure text format. You can break a large document into multiple files, and use nodes to bring them all together into a single book. Table of contents and index are automatically generated. Intra-document links (within arbitrary text, pointing to chapters or sections) are very easy. And with a push of a button, I can create a single-html-file version, a chunked-html version (one file per chapter), and a PDF version.
After some tweaking and customization, I'm very happy with the output. The documents look great!!
DocBook is used extensively by real publishers (most notably, O'Reilly), and it's been around for more than fifteen years, so it's reached a certain level of maturity.
On the other hand, all of the processing is done with XSLT, using an ad-hoc collection of tools. (My own docbook pipeline includes Python, Java, Xerces, Xalan, Apache FOP, and PDF-SAM. Plus the official XSLT stylesheet distribution, and my own XSLT customizations.)
DocBook is not a turnkey solution. You won't be able to get going quickly, without reading the manual. And if you don't know anything about XSLT, you'll have to learn.
On the other hand, there are only a dozen or two XML tags that you really need to know to write the documents. (The real expertise comes into play during doc generation from the XML sources.) If one person on your team was willing to be responsible for writing the doc build script, then everyone else on the team could just learn the DTD and do a decent job contributing.
Anyhow... DocBook definitely has some faults. It's not the easiest system for tech authorship. But it's the best open source tool I know of.
The "Subversion Book" is written in DocBook. Here's a page with links to the different book versions (single-html, chunked-html, and PDF):
http://svnbook.red-bean.com/
And here's a link to the DocBook XML sources for the first chapter, so that you can get an idea for how it works:
http://sourceforge.net/p/svnbook/source/HEAD/tree/branches/1.7/en/book/ch01-fundamental-concepts.xml
