These are things I see popping up a bit these days. I like the way they expose their intent through the "fluency" of the language of the statements.

For example, I've been used to writing asserts in my unit tests like so:

   1: Assert.AreEqual("This thing", result);

And I've just tried the fluent interface exposed by NUnit along with one of the provided methods of the string class:

   1: Assert.That(result.Equals("This thing"));

It achieves exactly the same result as the first example, but it reads a lot cleaner to me. I just hope I don't start writing lots of unnecessary fluent rubbish:

   1: Increase.The(principalBalance).By(theTransaction.Amount);
Hmm...