• XmlSchemaNotes
• XsltNotes

http://www.snee.com/xml/nscomments.html - an XSL stylesheet that shows the namespace values of each element and attribute in a document.

I've been burned by offline systems getting fussy when they couldn't load DTD's. Turns out I'm not alone: http://www.tbray.org/ongoing/When/200x/2005/12/15/Drop-the-Doctype . Bottom line: it's probably safer to use DTD's in development but drop them in production.

Emacs

nxml mode rocks: http://www.thaiopensource.com/nxml-mode/
Relax NG schemas for xslt 2.0: http://norman.walsh.name/2006/07/12/xslt20
Relax NG schema for XML Schema: http://www.jenitennison.com/schema/xmlschema.rng