install it in web.config:
<xslExtensions>
...
<extension mode="on" type="Sitecore.Web.WebUtil, Sitecore.Kernel" namespace="http://www.sitecore.net/webutil" singleInstance="true" />
</xslExtensions>
declare it in your xslt:
xmlns:webutil="http://www.sitecore.net/webutil"
then use it (example from a "print page" footer):
<xsl:text>Artiklens placering: </xsl:text><xsl:value-of select="webutil:GetServerUrl()" /><xsl:value-of select="sc:path(.)" />