I'm thinking about works as close to books as possible. Please feel free to add comments, as well as options I may be missing.<p>(I realize the choices I'm giving focus on the markup language rather than the tool (e.g. <i>reStructuredText</i> rather than <i>Docutils</i>), and that I'm mixing software products (such as MS Office) with markup language specifications, but please bear with me.)
I use reStructuredText as it seems fairly complete in terms on functionality and yet very simple to read as plain text. However, unless we talk about well-established systems such as TeX and LaTeX, I fear about long-term availability and support with so many ongoing projects.