WYMeditor is a web-based WYSIWYM (What You See Is What You Mean) XHTML editor (not WYSIWYG).
WYMeditor generates perfectly structured XHTML strict code, to conform to the W3C XHTML specifications and to facilitate further processing by modern applications.
This editor is supported by almost all browsers and it has been integrated in many open and commercial applications, such as Drupal, Symphony, Django, Rails etc.
Download: http://www.wymeditor.org/download/
Demo: demo
License: MIT and GPL licenses