For some time we were using DocBook, but it was very hard to extend with more advanced, and necessary features (syntax highlighting, splitting into several files, multilinguality management etc.). Later, we decided to write our own system from scratch and release it as open-source: link text. It uses plain text files and Markdown as the syntax language, and now we have everything we need. The disadvantage is that currently there is probably no Markdown parser that produces something else than HTML output. For now this is enough, but we are thinking about implementing PDF support quite soon.
Moreover, we are maintaing MediaWiki as a community-based help.