XSLT Development Tools
Here are a couple of interesting tools to simplify XSLT development (courtesy of the Metaphorical mailing list):
XSLT-Process – XSLT-process is a minor mode for GNU Emacs/XEmacs which transforms it into a powerful editor with XSLT processing and debugging capabilities.
Alchemist XML IDE – This commercial product features an XPath expression evaluator, XPath generator, XSLT Editor, an XSLT debugger, and much more. Free 15-day trial available at http://www.mentattech.com/themes/mentat/download.html