Chapter 13 of this book discussed how developers can write their own custom extensions to Ext JS. Being an open source project, the Ext JS community is very active in contributing new components to extend existing functionality, and sometimes these components even go on to become a part of the framework.
Many custom extensions are found directly in the forums area of the Ext JS website, as posts from members who have come up with solutions to a problem they had identified. There is, however, a section of the Learning Center area of the site that is dedicated to showcasing custom extensions. Here are a few fan favorites: