public static string Menu(this HtmlHelper helper, string siteMapProviderName)
        {
            var provider = SiteMap.Providers[siteMapProviderName];
            return NodesAsHtml(provider.RootNode);
        }

        private static string NodesAsHtml(SiteMapNode node)
        {
            var writer = new StringWriter();

            if (node.HasChildNodes)
            {
                if (!String.IsNullOrEmpty(node.Title))
                {
                    writer.Write("<span>{0}</span>", node.Title);
                }
                writer.Write("<ul>");
                foreach (SiteMapNode childNode in node.ChildNodes) 
                {
                    writer.Write("<li>");
                    writer.Write(NodesAsHtml(childNode));
                    writer.Write("</li>");
                }
                writer.Write("</ul>");
            }
            else
            {
                writer.Write("<a href=\"{0}\">{1}</a>", node.Url, node.Title);
            }

            return writer.ToString();
        }