The webinar on how to create reusable validation attributes with your own custom validation logic is available here:
You will learn how to create a custom validation attribute, and how to perform server-side as well as client-side validation including the code needed to use the unobtrusive validation plugin for jQuery.
I will show you how to create a RequiredWhen attribute which makes the property only required when another property has a specific value.
The source code of the project is available on GitHub.
Let me know what kind of custom validation attributes you have created!