You can also check out AxKit (the site has been down as the author is moving but should be up again soon), an Apache perl_mod module. It's written (obviously) in Perl and provides out-of-the-book support for automatic DocBook rendering to HTML (in a couple of forms).

It uses XSLT and XPathScript to do its dirty work (using the XML::LibXML and XML::LibXSLT modules), which is nice since you can actually easily integrate multiple XML markups (you could generate product specific or task specific ones, like todo lists that are much lighter weight than DocBook) by simply assigning different stylesheets and people looking at the output are none the wiser :-)

There is also an XML markup aimed at simulating LaTeX (someone else mentioned LaTeX) though its name currently escapes me. I use LaTeX on a pretty regular basis, its fine for things like papers (nothing beats it for mathematical formulae), but it doesn't really have good facilities for web delivered documentation---it was meant for a static print world and it performs that task admirably.