An outliner, written in Python, with a difference. See http://buzz.sourceforge.net . Also, see the following comment by BillSeitz in GeneralDiscussion200204 :

Bill Seitz <fluxent@yahoo.com>, 2002/04/30 22:01 GMT (via mail): An idea I've had for editing longish structured individual pages: use the buzz outliner, which is written in Python and uses OPML as its native format, and then in Zope write code for translating between OPML (which is brain-dead simple) and StructuredText. Dealing with the stx features that don't map to OPML well (e.g. tables) could be ugly... (of course this ties you to a machine where Buzz is installed...) Someday...