Since the audience of this blog is interested in App Dev, I wanted to include some additional information not directly related to Composite Applications. The sidebar and toolbar can be extended to provide new features.
Jo Grant posted an entry describing the Usage of ViewParts in Composite Applications and Sideshelf. This described how to create one view that can be used as both a shelf view in the sidebar and a component.
To learn more about the sidebar and toolbar, you can review Extending the IBM Lotus Notes V8 sidebar and toolbar. Brian Leonard describes two samples: SideNote sidebar application and QuickNote toolbar application.
Once your plug-ins are built, they can be deployed using information in Documentation -- Sidebar Contribution Design Pattern and Migrating a Sametime 7.5 plugin over to Notes 8. In addition to describing how to use Sametime 7.5 and 7.5.1 plug-ins in Notes 8, it also describes in the last section how plug-ins can be installed.
If designed correctly, plug-ins developed for Notes can be reused and integrated into different parts of the UI (e.g. Composite Applications, sidebar, toolbar) or in any applications based on the Lotus Expeditor platform.