What libraries or frameworks to create custom Linq provider do you know?
How's you experience? Pros and cons? What's the best?
Take a look at Re-linq (http://relinq.codeplex.com/). NHibernate uses it in its Linq provider in 3.0 version. We used it too in out project to simplify linq expressions for serializing. I think everyone, who wants to create Linq provider, should take a look on it. Also this article by Ayende Rahien can be useful.