merb-slices are “Little slices of MVC cake”. These are self contained merb apps with models, controlers, views and assets that you can distribute as rubygems. You can mount a merb-slice at a specific point in your router definition and you can override any part of the slice up in your main app. So in a way these are similar to what Rails-Engines promise, except merb-slices are built into the framework and will not break when merb itself is updated.
Check out the tutorial/screencast for a peek at how merb-slices work.
[Nice. Merb continues to impress.]
Source: Brainspl.at