Gonzalo Ayuso has a recent post to his site showing you how to use Stack to create bundles for your Silex-based applications. Stack is a structure that allows for the reuse of HttpKernelInterface middleware to create a custom "stack" of functionality.
Nowadays almost every modern framework/applications implements HttpKernelInterface (Symfony, Laravel, Drupal, Silex, Yolo and even the framework that I'm working in ;) ) and we can build complex applications mixing different components and decorate our applications with an elegant syntax. The first thing than come to my mind after studying Stack is to join different Silex applications in a similar way than Symfony (the full stack framework) uses bundles.
In his example, he shows an example of a simple GET endpoint for a blog and API. In the first version, he calls the "run" method to execute the handling of the request. The second example, however, includes the attachment of a simple "MyKernel" middleware to the stack and calls "handle" to apply and send the modified Response back to the user.