MVC3 Razor JQuery client side validation with an additional alert box
public class LogonViewModel
    {
        [Required( ErrorMessage = "User Name is Required")]
        public string UserName { get; set; }

        [Required( ErrorMessage = "Password is required")]
        public string Password { get; set; }
    }

Html.ValidationSummary()

@Html.ValidationSummary()
 @using (Html.BeginForm(null, null, FormMethod.Post, new { id = "loginForm" }))
     {
      username: @Html.TextBoxFor(m => m.UserName) <br/>
      password: @Html.TextBoxFor(m => m.Password) <br/>
      <input type="submit" value="Login"/>


    }

<script language="javascript">
        $(document).ready(function () {

            $("#loginForm").validate({
                invalidHandler: function (form, validator) {
                    var errors = validator.numberOfInvalids();
                    if (errors) {
                        alert(errors);
                    }
                }
            });
        });



    </script>

$(document).ready(function () {
            $("#loginForm").bind('invalid-form.validate',
                function (form, validator) {
                    var errors = "Login failed:rn";

                    for (var i = 0; i < validator.errorList.length; i++) {
                        errors = errors + "rnt-" + validator.errorList[i].message;
                    }

                    alert(errors);


                }
                );
        });