I've put together a module that integrates the edit-on Pro WYSIWYG editor with Drupal. edit-on Pro is a full-featured cross-platform (Java) editor that outputs clean accessible XHTML. Unfortunately it isn't free/open source. Pricing starts at €249 for a 10 seat license.

There is still a fair bit of work to do. For instance the module currently lacks support for image uploads.

Update: A permissions typo locked out everyone but user 1. I've also put up a demo site.