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: 
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.
You can also checkout the [.net validation framework](http://www.codeplex.com/ValidationFramework). Its a rules framework that lets you create validators, apply the validators to rules, attach rules to your model, and check those rules at runtime on both the client and server. It provides flexible ways to configure rules - making heavy use of linq for both fluent and strongly typed configuration. It also provides extensibility points to create your own client script generators and rules.

The framework leverages the MVC RC HtmlHelpers and default conventions.

If you download the latest source you can see an example of the framework working in the SplitBranch -> QSAspMvc quickstart project. Its still being actively developed.