BitFlux, an open source Wysiwyg XML editor
Bitflux Editor is a browser based Wysiwyg XML Editor – and that changes everything! You can edit now your content semantically and at the same time display it to your users and editors in its final form.
That's a big stone in the garden of old school text editors.
Note to Mac users: it does not work in Safari. Try Mozilla 1.3+ or Netscape 7.