You can use javascript for that also.
<script type="text/javascript">
function alerttest()
{
var text = document.getElementById('<%=TextBox1.ClientID%>').value;
alert(text);
return true;
}
</script>
<asp:Button ID="btnAlert" runat="server" OnClientClick="return alerttest();"
Text="Alert" UseSubmitBehavior="false"/>
Now check this one.