Extensible Stylesheet Language (XSL) 1.0

XSL is a language for expressing stylesheets. It consists of two parts: a language for transforming XML documents, and an XML vocabulary for specifying formatting semantics. (W3C Recommendation 15 October 2001)
http://www.w3.org/TR/xsl/

Unofficial MSXML XSLT FAQ

This is a simple URL that people can use when getting started with XSLT and IE. 19 FAQ (frequently asked questions) which are answered in great detail and with plenty of suggestions to resolve problems.
http://www.netcrucible.com/xslt/msxml-faq.htm

XML Cover Pages: Extensible Stylesheet Language (XSL)

Introduction, history, articles and software support by Robin Cover. (Oasis)
http://www.oasis-open.org/cover/xsl.html

TopXML XSLT Zone

With 34 original articles, the XSLT Zone explains XSLT in various solutions - such as .NET, mobile solutions and in applications. These articles range from introductory to advanced; through to tutorials and include complete XSLT and XPath references.
http://www.topxml.com/xsl

XSLT Requirements Version 2.0

This draft document describes the requirements for the XSLT 2.0 specification. (W3C Working Draft 14 February 2001)
http://www.w3.org/TR/xslt20req

Jeni's XSLT Pages

Jeni Tennison is a very active XSLT community member. This page is her homepage for her many XSLT activities. With 25 tutorials/articles and 2 utilities for visitors. Good website.
http://www.jenitennison.com/xslt/index.html

XSL Frequently Asked Questions

A list of FAQ covering both current and upcoming topics. The questions range from basic (how to handle special characters) to advanced FO and XPath specifications which as yet are not standards. An outstanding site.
http://www.dpawson.co.uk/xsl/xslfaq.html

Marrowsoft Xselerator

Xselerator is a fully functional XSL/XSLT IDE and Debugger - with a clear and easy to use IDE offering element and attribute automatic tag completion!
http://www.vbxml.com/xselerator/

XSL Transformations (XSLT) Version 1.0

This specification defines the syntax and semantics of XSLT, which is a language for transforming XML documents into other XML documents. (W3C Recommendation 16 November 1999)
http://www.w3.org/TR/xslt

MSDN's XSLT Developer's Guide

The successor to Extensible Stylesheet Language (XSL), XSL Transformations (XSLT) is an XML-based language that enables you to transform one class of XML document to another
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/xmlsdk30/htm/xmconxsldevelopersguide.asp

NIST Toolbox

Collection of tools to make enable XML applications to interchange data easier. Includes APEX and ATTS.
http://ats.nist.gov/xsltoolbox/

UWOBO XSL Processor

Invoked via URL giving the URI of an XML document and a list of stylesheets to be applied. Requires Tomcat and Xerces/Xalan.
http://www.cs.unibo.it/helm/uwobo/

Jade

An early implementation of an XSL processor. Converts XSL to DSSSL to render XML.
http://www.cogsci.ed.ac.uk/~ht/xslj.html

XSL Lint

A semantic lint checker for that detects poor syntax and redundant areas of a stylesheet.
http://www.nwalsh.com/xml/xslint/