This is a demo application that shows different data validation methods in ASP.NET MVC 4:

• Adding validation errors directly to the ModelState
• Applying validation attributes
• Implementing model validation with IDataErrorInfo
• Implementing model validation with IValidatableObject
• Implementing client side validation
• Implementing remote validation

It is accompanied by a tutorial posted on my professional blog: http://codeutil.wordpress.com/2013/03/25/mvc-data-validation/