Remote validation, a new feature in MVC 3 for single field server side validation

Common scenario like, you want to check a user name if exist before posting all of information to server.

Before MVC 3, this kind of validation can be implemented by either JQuery ajax calling an action in MVC or a web service in webform.

ASP.NET MVC 3 provides a mechanism that can make a remote server call in order to validate a form field without posting the entire form to the server

http://msdn.microsoft.com/en-us/library/gg508808%28v=vs.98%29.aspx