Microsoft's XML Developer Center 10/27/99 Stars5.gif (1046 bytes)
Huge, huge library of XML information.
XMLSchema.com 2/8/00 Stars5.gif (1046 bytes)
"This preview of Extensibility's XMLschema.com™ provides you with the latest facilities for testing your schemas and converting them to your preferred XML schema dialect." Also has white papers that describe best practices for creating XML schemas, etc. Extensibility also makes XML Authority, a great application for building XML schemas.
Cafe con Leche XML News and Resources 2/8/00 Stars5.gif (1046 bytes)
XML resources, from a non-Microsoft (almost anti-MS) point of view. It's a useful site, but the articles aren't generally very well-written. Maybe they're written by grad students?
XML.org 1/24/00 Stars5.gif (1046 bytes)
"XML.org serves as a reference for XML vocabularies, DTDs, schemas, and namespaces." Yes, it's just that dull. But it may be important if you're trying to come up with an XML standard for a particular industry.