class Library
  Plugins = []
  
  def initialize#(...)
    Plugins.each do |plugin|
      # Here you can run a certain class method or grab some data from each class
    end
  end
  
  # ...
end

class LibraryPlugin
  # ...
  
  def self.inherited(sub); Library::Plugins << sub; end
end

class LibraryFooer < LibraryPlugin
  
end
class LibraryBarer < LibraryPlugin
  
end

Library::Plugins.inspect #=> [LibraryFooer, LibraryBarer]