Because Blacklight is a configurable and extendable Ruby on Rails engine, other plugins exist for Blacklight that add additional functionality. These plugins themselves are Ruby on Rails engines that usually require Blacklight as a dependency and inject custom behavior.
Often these plugins fall into two different categories: a plugin that provides custom functionality or a plugin that exists as a certain type of domain specific Blacklight application.
There are many more of these plugins that exist. An example of this would be Blacklight Range Limit which adds range limit slider facets to the facet views. These types of plugins are often used in conjunction with eachother.
These types of plugins provide a specific type of Blacklight application experience. Three of these exist including:
Blacklight plugins exist as separate software projects, and can even have adjacent communities that help maintain them.