Excellent introduction to XSLT

Rescuing XSLT from Niche Status is A Gentle Introduction to XSLT through HTML Templates. It is something of a hybrid article, consisting of a discussion of the problems involved with both teaching and learning XSLT followed by an excellent (if somewhat brief) tutorial covering the most important XSLT tags and concepts and how they can be used to convert a simple XML document in to XHTML.