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.

I've finally decided to roll out version 1.0 of my XStandard module for Drupal.

So why choose XStandard over FCKeditor or TinyMCE? Simply put, it produces valid markup. The pro version can actually filter material pasted from Word into semantic XHTML while retaining structure. Here's a full feature list.