The thing I'm most impressed with in NHibernate's mapping-by-code feature is its API design. Fabio Maulo, its creator, calls this a loquacious interface, as opposed to chained, fluent interface. It's an API intended to build complex structures in code that resembles the structure itself. I don't know any other projects using solely this kind of API either. But I think this is going to change soon, as Fabio's approach seems to be more powerful and in a lot of cases more readable and "fluent" than chained interfaces.