A trivial CoffeeScript.org -> Javascript plugin for jekyll ( https://github.com/mojombo/jekyll ). Put this file in '_plugins/' and write a YAML header to your .coffee files (i.e. "---\n---\n")

coffeescript_converter.rb

module Jekyll
  require 'coffee-script'
  class CoffeeScriptConverter < Converter
    safe true
    priority :normal

    def matches(ext)
      ext =~ /coffee/i
    end

    def output_ext(ext)
      ".js"
    end

    def convert(content)
      begin
        CoffeeScript.compile content
      rescue StandardError => e
        puts "CoffeeScript error:" + e.message
      end
    end
  end
end

According to this post on SO OctoPress HTML escapes the output.

This fork fixes that OctoPress issue: https://gist.github.com/2925325

EDIT: The above fork is only needed on older versions of Octopress. Latest works with this gist

If you want a more comprehensive asset pipeline solution, you might consider Jekyll Asset Pipeline which will convert CoffeeScript, Sass, Less or any other language via an extension. It can also compress assets via Yahoo's YUI Compressor or Google's Closure Compiler.