I got this from a friend and have found it very useful for debugging my XSLT from content query web parts and data view web parts in Visual Studio. The below XSLT will output the source XML given to it:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt"
exclude-result-prefixes="msxsl">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="@ node()">
<xsl:copy>
<xsl:apply-templates select="@ node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>