- docextract - extracting doctools text from source files
- docstring using leading comment in proc bodies
- docstrip and tclldoc - Literate programming the scripting way
- doctools - this is more a generic markup language tool
- doxygen - documentation tool; has tcl support starting with version 1.7.5 (see doxygen+tcl) and can be used for tcl with a filter/pre-processor   A little doxygen converter
- HelpSystem interactive help doc embedded into tcl/tk code - kind of different
- Ruff! generates documentation via runtime introspection
- Simpledoc  - very simple: generates HTML from source with markup in comments.
- tcldoc - similar to javadoc
- TL - this is a glue language that has auto documentation functions
- TMML - this is more a generic markup language tool
- tycho - this GUI itcl dev environment included a system to create HTML from itcl classes
- feathers: an hand-written documents tool
- NaturalDocs - attempts to parse code docs written in a very natural manner
- XOTcl Documentation Tool
- ParseTools - extension that can be used to write documentation extractors
- The Simple Library - framework to support nearly literate programming doc
escargo 21 Apr 2005 - One thing to keep in mind with these tools is that some of them might be used with OO systems that extend the Tcl syntax, Snit for example. Now, instead of just need to track proc definitions, namespaces, and global variables, you might want to keep track of classes, methods, typemethods, and other interesting extended Tcl syntax. I haven't looked at all of these tools, but I know that robodoc made it possible to handle pretty arbitrary things to track and document.
MAKR, 2006-12-08: Half a year ago I decided to move over to robodoc. I now have all in-source documentation (C and Tcl) converted to this format. This was one of the best ideas I ever had. The commentary in the sources is clean and readable. The resulting HTML documentation is it too. You can format it to your liking using CSS...escargo - Do you use any OO extensions, or just plain Tcl/Tk?MAKR, 2006-12-13: As it gets tool specific, I continue at the robodoc page ...
MAKR, 2007-12-14, just stumbled over discussions about the Tcl user documentation project and also saw an URL  at c.l.t. about how to write manpages.
Tcl Style Guide has a couple of recommendations about documentation near the end of the page.
There is, on the wiki, a page about docbook, however, I'm uncertain how much use of docbook occurs in the tcl development community efforts to document tcl.
GSoC 2011 has a project proposal in this direction: GSoC Idea: Auto Documentation Tool for Tcl
Lorance: Since I have used Perl POD for years I decided I Should use that for Tcl. I have added embedded POD support to my personal tcl.vim syntax file. See my Github Repository for more information (The Wiki has screenshots).