Validation is such an important part of development, and the best way I have found for the MVC is by Steven Sanderson, xVal, which is a validation framework for ASP MVC, even Scott Gu thinks it is absolutely fantastic.

If you are after using xVal with Linq to SQL then it is well worth checking out Integrating xVal Validation with Linq-to-Sql by Adam Schroder

It’s worth checking out Client-side form validation made easy by Adrian Grigore to extend this validation even further.

Don’t take my word for is, check out, ASP.NET MVC Validation Refresh: Best Techniques & Frameworks by Graham O’Neale