Doctools is a simple Tcl-based markup language suitable for writing software documentation. (Here is the language introduction and here is the command reference.) The doctools package distributed with Tcllib (and present on Mac OS X 10.8) provides an implementation that can transform Doctools markup to a variety of formats, including HTML and nroff.
BBEdit is a Macintosh text editor that doesn't suck.
This repository provides a few tools to help write documentation with Doctools in BBEdit.
The Doctools.plist file is a language module that provides basic syntax highlighting of Doctools commands.
The DefaultFilter_Doctools.tcl file is a Preview Filter for BBEdit 10.5, used to generated content for BBEdit's Preview window while editing Doctools files. It reads Doctools markup from standard input and prints the corresponding HTML to standard output.
Install Doctools.plist in ~/Library/Application Support/BBEdit/Language Modules/.
Install DefaultFilter_Doctools.tcl in ~/Library/Application Support/BBEdit/Preview Filters/.
(You may need to create the Language Modules or Preview Filters folders if they are not already present.)
These files are in the public domain.