This site is dedicated to all things XML. Of course, there are all sorts of other pages around that cover XML. Jeni's XML Pages, though, are dedicated to helping people understand and make the most of using XML.
I'm working on this site all the time, so do check back here again, and let me know if you'd like anything particular added.
Some pages about XSLT, including tutorials.
Some pages about the freelance services that I offer.