Question: Does ASP.NET MVC provide a standard validator functionality or do you have to create your own validation manually? If the latter, is there any third party validator available that you can use on ASP.NET MVC web applications?

Answer: 
Shortly after I posted this answer I found [xval](http://xval.codeplex.com/) which is a validation framework for ASP.NET MVC.
ASP.NET MVC contains methods like `Html.ValidationSummary()` and `Html.ValidationMessage()`. These are updated automatically if you use `TryUpdateModel`. You could also validate manually and set the errormessages yourself. [Here](http://blog.maartenballiauw.be/post/2008/08/29/Form-validation-with-ASPNET-MVC-preview-5.aspx) is an example of how use it.