Syntext Serna v2.0, a "WYSIWYG" XML editor for Windows, Linux and Mac OS X. Serna renders your XML through your XSL stylesheets on the fly as you type, supposedly giving you a WYSIWYG editor experience for any structured content. I assume custom DTDs would require custom XSL, possibly specially written for the purpose of editing, but it comes with out-of-the-box support for DocBook and DITA.