HOW TO: Implement Validation in ASP.NET MVC

Performing Simple Validation (C#) by ASP.NET Team - using ModelState object.
Validating with the IErrorDataInfo Interface (C#) by ASP.NET Team - using IErrorDataInfo interface.
Validating with Data Annotation Validators by ASP.NET Team - using Data Annotations Model Binder.
See also: ASP.NET MVC Tip #43 - Use Data Annotation Validators by Stephen Walther
ASP.NET MVC Tip #42 - Use the Validation Application Block by Stephen Walter - demonstrates how you can use the Microsoft Enterprise Validation Application Block within an MVC application to perform both basic and advanced form validation. The Validation Application Block supports a rich set of validators that you can begin using in an ASP.NET MVC application.
xVal - a validation framework for ASP.NET MVC by Steven Sanderson - xVal is a validation framework for ASP.NET MVC applications. It makes it easy to link up your choice of server-side validation mechanism with your choice of client-side validation library.
ASP.NET MVC Tip #3: Client-side form validation made easy - Part 1 by Adrian Grigore (devermind.com) - using xVal.
Creating Client and Server-Side Form Validation using the Validator Toolkit for ASP.NET MVC by Jurgen Baurle (CodeProject) - The Validator Toolkit is a simple and easy way to add client-side and server-side HTML form validation to the new ASP.NET MVC Framework. It is easy to extend the toolkit by adding custom validators.