<ajax:TabContainer ID="TabContainer1" runat="server" CssClass="fancy fancy-green">
        <ajax:TabPanel ID="tbpnluser" runat="server">
            <HeaderTemplate>
                New User
            </HeaderTemplate>
            <ContentTemplate>
                <asp:Panel ID="UserReg" runat="server">
                    <table align="center">
                        <tr>
                            <td>
                            </td>
                            <td align="right">
                            </td>
                            <td align="center">
                                <b>Registration Form</b>
                            </td>
                        </tr>
                        <tr>
                            <td>
                            </td>
                            <td align="right">
                                UserName:
                            </td>
                            <td>
                                <asp:TextBox ID="txtuser" runat="server"></asp:TextBox>
                            </td>
                        </tr>
                    </table>
                </asp:Panel>
            </ContentTemplate>
        </ajax:TabPanel>

<script type="text/javascript">
    function alert() {
        var value = document.getElementById("TabContainer1_tbpnluser_txtuser").value;
        alert(value);
    }
</script>

<asp:TextBox ID="txtuser" onblur="alert();" runat="server"></asp:TextBox>

var value = document.getElementById('<%=TextBoxID.ClientID%>').value;

document.getElementById("TabContainer1_tbpnluser_txtuser").value;

var value = $('#<%=txtuser.ClientID%>').value;
alert(value);