Observer

If you've ever used the MVC pattern, you've already used the observer design pattern. The observer pattern is like the View part of MVC. You have a subject that holds all of the data and the state of that data. Then you have observers, like users, that will pull data from the subject when the data has been updated.

There are so many applications for this design pattern that it became an integral part of MVC. Sending user notifications, updating, filters, and handling subscribers can all be done using the observer patter.