Spec#
Spec# is a extension to C# which add contracts to the language. Contracts are nothing new, it has been around for decades, but aren’t present in most popular languages. In my opinion this is really sad, because it prevents lots of unnecessary and simply bugs in programs.
According to the video, Microsoft uses contracts for example the windows source code (Contracts for c).
Watch the talk about Spec# on Channel 9. Its really worth it =).
http://channel9.msdn.com/Showpost.aspx?postid=406626