gistfile1.rb

# In Jekyll core...

module Jekyll
	module Filters
		class CustomFilter
			cattr_accessor :extensions
			@@extensions = []
			def self.extended(base)
				@@extensions << base
			end
		end
	end
end

# Definition file...
module Jekyll
	module Filters
		class MyFilter < CustomFilter
			... methods ...
		end
	end
end

#Later....
Jekyll.something(Jekyll::Filters::CustomFilter.extensions)