How to add ajax validation on field in ASP.NET MVC 3?
By Remote Attribute. This way is MS designed for you.

//In Model
        [Remote("CheckExistedEmail", "ControllerName")]
        [Required]
        [DataType(DataType.EmailAddress)]
        [Display(Name = "Email address")]
        [RegularExpression(@"^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,4})$",ErrorMessage="Invalid email address")]
        public string Email { get; set; }

//In Action 
        public JsonResult CheckExistedEmail(string Email)
        {
            if (EmailExisted(Email))
            {
                return Json("Existed Email.", JsonRequestBehavior.AllowGet);
            }
            else
            {
                return Json(true, JsonRequestBehavior.AllowGet);
            }
        }

http://msdn.microsoft.com/en-us/library/gg453903(v=vs.98).aspx

How many ways to redirect URL in Javascript?

1. window.location.href=”login.htm";
2. window.navigate(”login.htm”);
3. self.location=”login.htm”;

Any other way?

How to disable browser back button in asp.net?

Should make ASP.NET page cache expired instead of disable browser back button

Page.Response.Cache.SetCacheability(HttpCacheability.NoCache)

How to get active users in last 10 minutes in ASP.NET membership database?

select top 300 * from dbo.aspnet_Users u
where u.LastActivityDate > DATEADD(minute, -10, GETUTCDATE())
order by u.LastActivityDate desc

How to fix problem: missing files when publish a website in Visual Studio for ASP.NET?

In Visual Studiio, Content files need to be marked for publishing
Right click on the file missed -> Properties -> Build Action set to Content.