I am not sure about your question, but here is my solution
<script>
function openItem(ctrl)
{
window.open(ctrl.value);
}
</script>
<asp:listbox id="ListBox1" runat="server" onclick="openWebSite(this)" xmlns:asp="#unknown">
<asp:listitem text="http://www.google.com"></asp:listitem>
<asp:listitem text="http://www.yahoo.com"></asp:listitem>
</asp:listbox>