Let's take a look at a brand new MVC 3 validator - RemoteAttribute. The Remote validator is very simple to use and yet extremely powerful. Remote is for situations where it is impossible to duplicate server side validation logic on the client, often because the validation involves connecting to a database or calling a service. If all your other validation uses javascript and responds to the user's input immediately, then it is not a good user experience to require a post back to validate one particular field. This is where the remote validator fits in.
In this example, we are going to add remote validation to the username field to check that it is unique.

[Remote("ValidateUserName", "Account", ErrorMessage = "Username is not available.")]
public string UserName { get; set; }

Remote has three constructors allowing you to specify either a routeName, a controller and action or a controller, action and area. Here we are passing controller and action and additionally overriding the error message

public ActionResult ValidateUserName(string username)
    return Json(!username.Equals("duplicate"), JsonRequestBehavior.AllowGet);