Portals on XML

http://www.w3.org/

http://xml.startkabel.nl -- news, links

http://zvon.org -- excellent collection of tutorials, on-line references in many languages, hosted in CZ

xml.coverpages.org -- daily updated collection of links to articles, standards, software, etc. in XML. Best in this category.

http://xml.com -- articles, tutorials at a high level

http://ibm.com/developer/xml -- papers, tutorials, software atd. at a high level