Internet Explorer actually works as a great debugger for XSLT.

http://www.microsoft.com/downloads/details.aspx?FamilyId=D23C1D2C-1571-4D61-BDA8-ADF9F6849DF9&displaylang=en

This will let you step through transformations from a provided XML file:

http://www.microsoft.com/downloads/details.aspx?familyid=2fb55371-c94e-4373-b0e9-db4816552e41&displaylang=en

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