The Observer Pattern.
Its easily the coolest and most practical pattern I have ever used and since I am big fan of DRY (Don’t Repeat Yourself), I will redirect you to an article which I recently wrote on CodeProject about the Observer pattern.
The above link has different techniques (along with source code) of implementing Observer pattern in .NET using C#.
It explains the various libraries provided by Microsoft in successive versions of .NET framework in order to implement the Observer Pattern.
The Observer pattern a.k.a Publisher-Subscriber pattern is the one to which you can actually relate in the real world.
Hope its an interesting read.