Apache Software Foundation: http://www.apache.org
Essential XML Quick Reference , by Aaron Skonnard and Martin Gudgin (Boston, MA: Addison-Wesley, 2002). I don't directly recommend very many other books in this book, but this is one I suggest without reservation. If you deal with XML on any technical basis at all, this book needs to be in your collection.
Microsoft's XML Resources: http://msdn.microsoft.com/xml
Sun Microsystem's Java Technology and XML: http://java.sun.com/xml
World Wide Web Consortium: http://www.w3.org
The XML Cover Pages: http://xml.coverpages.org