Jekyll markdown references plugin by Olov Lassus: Keep all your markdown reference-style link definitions in one file (_references.md)

references.rb

# references.rb: jekyll markdown references plugin
#
# add this file to your _plugins directory (create it if needed)
# create a file (exactly) named _references.md in your Jekyll site root,
# then add your markdown reference-style link definitions to it.
# for example:
#   [jsshaper]: http://jsshaper.org  "an extensible framework for JavaScript syntax tree shaping"
#
# you can now reference these links in any markdown file
# for example:
#   You should [check out JSShaper][jsshaper]
# 
# 2012-05-20 Update by [mytharcher](http://github.com/mytharcher)
# - Move the module to the place under "MarkdownConverter".
# - Overwrite the "convert" method to insert a preprocess hook to append references content.
# - Add prefix and suffix new line to make sure that the references should be a separated markdown block.
#

module Jekyll
  module Converters
    class Markdown

      alias old_convert convert

      @@refs_content = nil

      def get_references()
        # read and cache content of _references.md
        if @@refs_content.nil?
          refs_path = File.join(Dir.pwd, "_references.md")
          @@refs_content = if File.exist?(refs_path) then File.read(refs_path) 
                           else "" end
        end
        @@refs_content
      end

      def convert(content)
        # To make sure that the references should be a separated markdown block.
        old_convert(content + "\n\n" + get_references + "\n\n")
      end
    end
  end
end

Hello! Just wanted to let you know that the new upstream for jekyll-references is at https://github.com/olov/jekyll-references