XML (eXtensibe Markup Language) is a magnet for hype: the successor to HTML for web publishing, electronic data interchange, and e-commerce. In fact, XML is just a notation for trees, little more than a verbose variant of Lisp S-expressions; and a way to define tree grammars, a poor-man's BNF. Yet this simple basis has spawned scores of specialized sub-languages: for airlines, banks, and cell phones; for astronomy, biology, and chemistry; for the DOD and the IRS. This note is a brief guide to web resources that explain XML, the associated core technologies, describes some representative applications and lists additional applications and resources.