Enamoured with Fluent Validation. So easy to use, well documented, intuitive. Plumbed right into ASP.NET MVC with an addition to Application_Start and an attribute on your model and yr good to go. I was up and running with the project in no time at all. Not all open source projects are like that. :)
I think the coolest part is the fluent interface, which allows super cool chaining logic. The When() and Unless() methods are quite handy as well, which makes creating complex logic when validating your model simple.
Was going to provide some code samples, but the project is so well documented that I don’t think that’s even necessary!