You need to write a manual that's more than a few pages. Maybe you'd like to be able to provide the client/customer/community with html as well as pdf. What format do you use, and why?
Depends on the type of documentation, but I'm a fan of LaTeX since there are so many tex2____ programs out there that'll convert LaTeX to whatever format you want.<p>The learning curve is steeper but the flexibility is high.
I currently use Scrivener, MultiMarkdown, TexShop, MacTex and OmniGraffle if I have a diagram or two.<p><a href="http://www.literatureandlatte.com/scrivener.html" rel="nofollow">http://www.literatureandlatte.com/scrivener.html</a> is a combination outliner/editor for OSX. Most of its features are focused on allowing you to manage a lot of different bits of a larger document. There's a full XSLT generation engine inside it so you can get it to generate just about anything you want if you're willing to roll up your sleeves a bit.<p><a href="http://fletcherpenney.net/MultiMarkdown" rel="nofollow">http://fletcherpenney.net/MultiMarkdown</a> is a simple markup syntax (derived from Markdown) which Scrivener supports.<p>Scrivener and MultiMarkdown together generate HTML or LaTeX from the base document, so you need to use something like <a href="http://www.uoregon.edu/~koch/texshop" rel="nofollow">http://www.uoregon.edu/~koch/texshop</a> and <a href="http://www.tug.org/mactex" rel="nofollow">http://www.tug.org/mactex</a> to get something worth printing.
The writers at my company use Framemaker - it's pretty industry standard and expensive. They have started writing the documents using DITA. It's pretty powerful.<p>DITA is an emerging documentation standard. It uses XML to describe the content and structure of a document. If you write your documentation in DITA XML, you can render it to PDF or online help easily. Yes, DITA is a little verbose.<p><a href="http://dita.xml.org/getting-started" rel="nofollow">http://dita.xml.org/getting-started</a>
there's an open source format (not software) that is pretty comprehesive (though at times repetitive): <a href="http://readyset.tigris.org/" rel="nofollow">http://readyset.tigris.org/</a>
<a href="http://docutils.sourceforge.net/" rel="nofollow">http://docutils.sourceforge.net/</a> is quite nice: A rich set of features, easily extensible by custom directives, Python, and a hell lot of export formats (LaTeX/PDF, HTML, S5 slideshows, ...).
POD. Learn it in 10 minutes. Generates all formats. Embeddable in code, if appropriate.<p><a href="http://perldoc.perl.org/perlpod.html" rel="nofollow">http://perldoc.perl.org/perlpod.html</a>