C# 3.0 & LINQ - Extensions Methods
"Extension methods are one of the many great new features of C# 3.0. They basically allow you to declare static methods that appear to be instance methods of a type, even if you have no access to the type (sealed classes, etc). The key point is that you don’t need to recompile or even have source access to the type you are working with. "
Check out http://blogs.msdn.com/erickt/archive/2006/09/12/751406.aspx