A trivial CoffeeScript.org -> Javascript plugin for OLDER OctoPress. Put this file in 'plugins/' and write a YAML header to your .coffee files (i.e. "---\nlayout:nil\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
        content = CoffeeScript.compile content
        # OctoPress will pass all content though a RubyPants filter.
        # RubyPants will conver quotes to smart quotes.
        # RubyPants will ignore any convertions when content is surrounded by HTML <script> tags
        # This adds HTML <script> tags to the output to prevent RubyPants from processing the JavaScript code.
        <<EOS
// <script> To prevent OctoPress filtering through RubyPants ([See Gist][1])
#{content}
// [1]: https://gist.github.com/2925325
// </script>
EOS
      rescue StandardError => e
        puts "CoffeeScript error:" + e.message
      end
    end
  end
end

HTML comments will fail if you have a '>' character in your source.

// <!-- ...
alert("barfoo");
if (blah > 0) {
    alert(&#8220;foobar&#8221;);
}
// &#8211;>

Use <script> tags instead. Be careful as the same rules apply as they do for embeded JavaScript inside HTML documents. You can not use the string </script> or you'll turn on RubyPants processing again. Instead escape this with "<"+"/"+"script>" (normal CoffeeScript string concatenation will not work here you must explicitly use the + operator to separate the characters and prevent parsing).

EDIT: Source patched to use <script> tags.

This Hack is no longer required for latest OctoPress. Use this gist instead.