Keyvan Nayyeri has a very cool example of using extension methods to add a Log method to all Exceptions.  The result is very cool:

  try
  {
    throw new ApplicationException("Shouldn't have gotten here...");
  }
  catch (ApplicationException e)
  {
    e.Log();
  }

Now I can catch any exception and easily get the message logged into my logging framework!  Admittedly, it isn't much different than calling a static method like my Log.Exception(e), but at first glance, it seems easier to remember...