Specflow

"I call this project 'specs', I didin't call this 'tests'. ... These are literally executable specifications ... and if you kind of think about that in a design sense, you usually write down the specifications of what you are going to build first; then you go ahead and build what you're going to build based on those specifications -- that in a nutshell is BDD.
Testing on the other hand, if you think about science, you might come up with a theory, then you might execute some tests against that theory ... testing usually comes later; I think that's a mental block for a lot of people."

http://specflow.org/specflow/screencast.aspx