I have been discussing using the Enterprise Library Validation Application Block for validation in the ASP.NET MVC Framework:
However, all bets are off on the best way to validate in the ASP.NET MVC Framework right now because the release candidate of the MVC Framework will support IDataErrorInfo and possibly have other changes that make most of the tutorials on the Internet on validation a moot point.
xVal - ASP.NET MVC Validation Framework
More interesting, however, is a validation framework by Steve Sanderson for the MVC Framework, called xVal, currently on CodePlex. This is a pluggable framework that supports both client-side and server-side validation. Currently it uses jQuery Validation for the client-side validation and pluggable solutions for Castle.Validator and System.ComponentModel.DataAnnotations for the server-side validation.
I haven't personally tried it yet, but the examples look really inviting.
I recommend checking out the announcement by Steve Sanderson and giving it a shot.