Extension Methods

"Extension methods allow developers to add new methods to the public contract of an existing CLR type, without having to sub-class it or recompile the original type.  Extension Methods help blend the flexibility of "duck typing" support popular within dynamic languages today with the performance and compile-time validation of strongly-typed languages."

 

http://weblogs.asp.net/scottgu/archive/2007/03/13/new-orcas-language-feature-extension-methods.aspx