Using ASP.Net MVC Validation Attributes outside of ASP.Net

Some people love them, others absolutely hate them, but if you ask me one of the coolest features in ASP.Net MVC is the Data Annotations that you use to decorate your ASP.Net MVC classes for validation. But one of the coolest things that came along in .Net 4.0 was support for this across the whole framework – even outside of ASP.Net MVC!