I have recently completed to develop the master pages topic in my ASP.NET course. You can find the community version of this course available for free personal and academic usage at www.abelski.com. More information about the professional version can be found at www.xperato.com. The following are few samples for the video clips I prepared for explaining this topic.
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.