Note: ASP.NET MVC 2.0 already includes a robust validation functionality and there's little reason to use this library in ASP.NET MVC 2.0. That said, if you are still stuck with ASP.NET MVC 1.0, this library may be helpful for you.
ASP.NET MVC Validation Library addresses the need of validating data rules for business entities at both server-side and client-side.
The library uses the Validation Application Block (VAB) included in the MS Enterprise Library 4.1
The code download include a sample application which helps you to understand how to use the library. The library code is commented well so you should be able to understand the API very quickly. If you don't want to read the source code to find out what it offers, check out the API information here