There are a couple of different .NET XSLT functions that I see used in the out of the box SharePoint web parts (RSS Viewer and Data View web part).
<xsl:stylesheet
xmlns:ddwrt="http://schemas.microsoft.com/WebParts/v2/DataView/runtime"
xmlns:rssaggwrt="http://schemas.microsoft.com/WebParts/v3/rssagg/runtime"
...>
...
<xsl:value-of select="rssaggwrt:MakeSafe($Html)"/>
<a href="{ddwrt:EnsureAllowedProtocol(string(link))}">More...</a>
...
</xsl:stylesheet>
Where can I find a reference that describes all of the extension functions that SharePoint provides?