I'm writing a manual for our new software product... and M$ Word just doesn't cut it. So what is the best software or language to use for creating/editing a software manual?
I would do it using latex. It can generate professional good-looking pdf and it helps you with table of contents, referencing images and sections, citations, etc.
I recommend texlive from http://www.tug.org/texlive/
I edit these files with vim, but there are some good gui softwares like LyX as Chris Charabaruk said and Texmaker, which helps you editing the source through its gui.
For a printed manual I go with LaTeX.
MikTex is the best Windows implementation in my opinion. My preferred editor is WinEdt
There is a great LaTeX tutorial by Tobias Oetiker
If it is going to be primarily an electronic manual and users will be online then a controlled wiki may be a good option.
You should ask yourself what features are important. Do you want a printed manual? An online manual with hyperlinks between sections? Links from the application into sections of the manual? Once you specify your needs in more detail people may be able to help further.
Dark Room for Windows, and WriteRoom for Mac are great programs in which to write and write and write. They run in fullscreen and eliminate everything but your text. So they're good for a first step, but not for formatting.
You could also use plain old HTML, especially if your manual is mainly for electronic reading.
At the risk of sounding unpopular, I'd really like to suggest using DocBook, along with a decent XML editor (preferably one that can crunch DocBook nicely). There is an XSLT toolchain available that can generate various different output formats including HTML or PostScript (which can be easily converted to PDF or printed).
While DocBook isn't the easiest thing to work with, it's a lot more powerful than simply using a Word document, and less complex and unwieldy than TeX.
As for a decent editor itself, LyX is a nice WYSIWYG editor for TeX documents, and includes some (outdated, unfortunately) support for exporting DocBook. I've seen some decent WYSIWYG editors for DocBook as well, but unfortunately their names aren't coming to mind at the moment; search and ye shall find.
EDIT: There's another SO question that asks for the best tools for DocBook, you might want to check the answers out on that one, too.
DITA for the content, DITA Open Toolkit for processing the documents, and then you have multiple options what editor to use, Epic, XMetal, Serna, oXygen etc.
We use Help and Manual. It's got some quirks, but it's pretty good overall; especially now that it saves in a non-binary format that can be merged. Your manual is in version control, right?
This Stackoverflow poster is asking much the same question and the answers contain a discussion of Framemaker and various alternatives in some depth. Disclaimer: I wrote the accepted answer.