What: NaturalDocs
  Description: Natural Docs is an open-source, extensible,
        multi-language documentation generator.
        You document your code in a natural syntax that reads
        like plain English.  Natural Docs then scans your code
        and builds high-quality HTML documentation from it.

See also robodoc - a tool very similar to NaturalDocs.

An example of a Tcl function documented in NaturalDocs:

  # Function: say_hello
  #   Say hello to the world
  # Parameters:
  #    name - who should we say hello to? 

  proc say_hello {{name "World"}} {
    puts "Hello, $name!"

NaturalDocs will pickup the proc line and place it in the documentation. It also works well with snit as it picks up the "method xyz {}" line, placing it in the resulting documentation. NaturalDocs also enables you to write more than source code documentation, i.e. general project documentation such as overviews, flow charts, goals and whatever else you can think of to place in an ordinary text file. Like robodoc it is cross-language.