An editor for ReStructured Text. I have recently added an editor for ReStructured Text to ETS.
As you can see in the screenshots below, the editor is largely composed of two panes, with the left pane containing an editor for the RST and the right pane displaying that RST as HTML. The HTML preview can be generated by vanilla docutils or by Sphinx, and this is configurable within the application. The HTML preview is updated live as you type. Sage: Open Source Mathematics Software.
Online reStructuredText editor.