Emacs Outline Mode

EmacsEditor has a built-in hierarchical mode which is a simple form of StructuredText.

EmacsWiki will tell you more about it: http://www.emacswiki.org/cgi-bin/wiki.pl?OutlineMode

The mode has many uses and many users. The minor mode (in conjuction with HTML mode, or anywhere you care to define some appropriate regexps for headings) is especially flexible.

This user often finds himself wanting to give people an EmacsOutlineMode file, and has to resist the urge to insist they view it in Emacs to get the benefit of the colours and folding. -- MatthewAstley

Outline mode is a remarkably general way to view structured text. For example, this configures outline mode to fold the different layers in properly-formatted HTML:

  (make-local-variable 'outline-regexp)
  (setq outline-regexp " *<[^/]")

EmacsOrgMode is built on top of this.