Quill is a free, open source WYSIWYG editor built for the modern web. With its expressive API, you can completely customize it to fulfill your needs.
Serna Free XML Editor
Serna Free XML editor is an easy-to-use WYSIWYG XML application intended for students, open-source project developers, and other non-commercial users. Indeed, Serna Free is the most powerful among free XML editors for WYSIWYG authoring.
Comes with an assortment of DTDs built-in and will publish to HTML and PDF. At first glance seems useful, but as with XML editors generally, if you are not familiar with the structure of the document you want to create it can be tough going.