My experiences of docBook, Doxygene, XML etc.
I work coding a huge system in a multi-language (FORTRAN, C, C# etc) environment. We also produce documentation of the stuff for users.Doxygen
I absolutely love Doxygen in combination with fx. C# for documenting the code. In C# the *same* documentation produces help-popups while typing (in Visual Studio and most likely in SharpDevelop) and html. This documentation can also produce xml, latex-code etc, etc. It's hard for me to see any other system producing all that outputs from the same source (on the other hand: I haven't tried all systems for documentation available). (I'll try to attach a screenshot to show what I mean.)
As I see it: any other system (or at least systems with documentation in one file, code in another file) will *always* have lags in the documentation. Having the documentation in the code is also a great motivation for commenting the code.
The only real trouble with doxygen as I see it is that documentation will look a little different if Emerald will be coded in a multi-language environment.
Having the documentation on-line in html/xhtml/text/yada-yada is excellent for searching and indexing. A pdf is typically slow to search.docBook
I once tried to use docBook but didn't even manage to produce hello world. docBook for me is eternally banned. wiki
I totally love the idea of wiki's - but as I see it wikis are not good for documenting code. A wiki for the faq, or something else could be great. (I have never really gotten into this kind of forum system.)Download
Downloading documentation can be chm-files or html or pdf, if Doxygen is used.
Please note that these are my experiences - interpret them as such - I do not claim to be almighty in my knowledge (but sometimes I think so).