I've been given an interesting task - figure out how to provide both client-side and server-side validation of forms, without duplicating the validation logic.
The ASP.NET MVC team just released Preview 5, which, among other bits of goodness, includes the ability to perform server-side validation automagically, and have the results of this validation sent to the view. It's pluggable, much like the rest of the framework.
Meanwhile I went in search of a validation framework (admittedly so I wouldn't have to write my own) and found ValidationFramework - which upon inspection appears to be exactly what I need for server-side validation. It's a well-put-together framework, and has an extensibility model that includes outputing client-side validation for ASP.NET using server controls.
Well, it seems I wasn't the only one to think so, as this discussion thread shows. I've been in contact with the original poster - Dane O'Connor - and have been reviewing the latest drops of a set of helpers and extensions on both ASP.NET MVC P5 and ValidationFramework that achieves exactly what I initially set out to do on my own!
Now I just have to hope that I'll be allowed by my employer to contribute some code to this project, as we'll be using the system in a production environment. Any improvements we provide would benefit others as well, and since the code is free, I'd like to give something back.
That's it for now.