Just published a new gem for my day job: Mixable Engines.
In the beginning, there was the engines plugin. This was back before Rails 2.3, when men were men and there was no “engines” functionality built into rails. The plugin would mix together the controller classes from your engines and your application.
In those days, my colleagues and I at the ASEE wrote lots of engines, and lots of apps that used those engines, and all of it depending on this mixed-together architecture. Now as we upgrade our apps to Rails 3, this gem allows us to keep the existing code and engines working without rewriting everything.