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.
I implemented a variant of the code I found on Stephen Walther's [blog](http://www.stephenwalther.com/blog/archive/2008/09/09/asp-net-mvc-application-building-forums-4-1-2-validation-revisited.aspx). I use it with LINQ2SQL models by defining an IValidatedEntity interface that includes the GetRuleViolations() method and implementing the partial OnValidate method that calls GetRuleViolations() and throws a custom exception if the number of violations is non-zero. In the controller, this fires on SubmitChanges for the data context. If I get an exception I requery the model via the GetRuleViolations() method to build model errors to pass back to the view.