Linq2vk - a linq provider to UserAPI
UserAPI is an official REST-like API for a popular Russian social network vkontakte.ru.
Technically Linq2vk is not really a custom LINQ Provider, it does not have an implementation of IQueryable interface. However it gives you a possibility to write the following code:
var friends = from f in api.MyFriends where f.IsOnline select f;
var updates = api.StatusUpdates.Take(20);
Linq2vk implements a lazy collection that will only request as much items as necessary.
Oh, and it's written in F#. Go check it out!