MSXML XSLT FAQ - Unofficial FAQ with answers to questions about XSL/XSLT on the Microsoft platform.
XSLT Test Tool - Tool that allows you to edit XSLT and compare the major XSLT processors side-by-side (windows only). The tool and installer currently support the following XSLT engines: Microsoft MSXML3, Saxon, Xalan, Oracle XSL, Sablotron, Xt, Unicorn, Napa, 4XSLT, and Instant Saxon.
XSLT Processor installer - Downloads and installs all of the major XSLT processors available and lets you get started using right away. Crude and simple, but usually works (windows only).
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Whitepaper - "Making the Web Semantic"