This is something that had been on my old blog in 2006 and went away, so I'm reposting something I wrote to the macsb list a few years ago:
For the BibDesk open source project, we wanted to have legit in-app apple help, as well as a web page (and PDF), without maintaining separate sources, since we were all working out of spare time.
We tried a few free options, and eventually worked out something decent with Texinfo, a latex-like markup language that can be processed into latex (and then PDF) or HTML.
I wrote a custom init file to have the (standard) texi2html script print out apple help compatible HTML. One post-processing script and a simple xcode script build phase later, and we get apple help.
We also get a version a little more suitable for the web (more content per page) from the same source: http://bibdesk.sf.net/manual/
It's worked well for a couple of years, and it's a good choice if you don't mind learning texinfo (it's pretty nice compared to writing in Docbook or HTML, IMO), and very good for version-controlled collaboration, since it's just text.
For an example, here's what the help source text looks like:
The init file and other stuff is here: http://bibdesk.svn.sourceforge.net/viewvc/bibdesk/trunk/bibdesk/BibDesk%20Help/
If anyone's curious about it, feel free to ask me questions here. I'll try to remember how it works.