NGenerics - "A class library providing generic data structures and algorithms not implemented in the standard .NET framework." This is cool code because it's not only adding new data structures, but also extending existing ones. They've added an IVisitor interface to add the Visitor pattern to create the VisitableHashTable amongst others.
More ...  - http://www.hanselman.com/blog/TheWeeklySourceCode5.aspx
Cheers - Dipesh