One interesting BDD frameworks is Concordion.NET. It is an open source BDD framework for the Microsoft stack that uses NUnit to execute the Concordion.NET tests: https://github.com/concordion/concordion-net
As Concordion specifications are written in simple HTML, it provides a good basis for a living documentation system. You can use a what-you-see-is-what-you-get (WYSIWYG) editor like BlueGriffon to describe your intended behavior of your software in simple HTML documents and instrument them to verify your system under test.
According to the excellent classification of BDD tools, Concordion.NET focuses on business readable input (and reaches business readable output as well). It moves even beyond BDD and supports ATDD: http://assertselenium.com/2012/11/05/difference-between-tdd-bdd-atdd/
