AssetsController. See README.txt.

README.txt

Assets controller for caching stylesheets and JavaScripts when deploying to Heroku or another read-only FS.
Stylesheets go in app/stylesheets/package/*.scss, JavaScripts go in app/javascripts/package/*.js.
Recommended packages are below, change them as you see fit - just make sure the folders exist.
Original implementation used Dir.glob to get all the files from a package folder,
but that soon revealed itself to be a folly as order is important with these things.

To include one of these packages in a view, use:
<%= cached_stylesheet_tag 'package_name', :media => 'media_type' %>
<%= cached_javascript_tag 'package_name' %>
for ERB, or
= cached_stylesheet_tag 'package_name', :media => 'media_type'
= cached_javascript_tag 'package_name'
for Haml. Both helpers pass all options along to the normal Rails helpers.

Thanks to this article for... everything:
http://avandamiri.com/2010/09/15/managing-styles-with-sass-on-heroku.html

application_helper.rb

def most_recent_date_in_folder(folder)
  Dir.glob(folder + '/*').map {|file| File.mtime(file).to_i}.max
end

def cached_stylesheet_tag(package, options = {})
  last_modified_date = most_recent_date_in_folder "#{Rails.root}/app/stylesheets/#{package}"
  full_css_path = stylesheet_path(package) + "?" + last_modified_date.to_s

  yield full_css_path if block_given?
  stylesheet_link_tag full_css_path, options
end

def cached_javascript_tag(package, options = {})
  last_modified_date = most_recent_date_in_folder "#{Rails.root}/app/javascripts/#{package}"
  full_js_path = javascript_path(package) + "?" + last_modified_date.to_s

  yield full_js_path if block_given?
  javascript_include_tag full_js_path, options
end

assets_controller.rb

class AssetsController < ApplicationController
  def index
    @output = ""
    package = params[:package].to_sym
    
    css_packages = {
      :screen => [],
      :print => [],
      :ie => []
    }
    js_packages = {
      :site => []
    }

    case params[:type]
    when :css
      if not css_packages[package].nil?
        sass_options = {:syntax => :scss, :style => Rails.env.development? ? :compact : :compressed}
        css_packages[package].map! {|sheet| "#{Rails.root}/app/stylesheets/#{params[:package]}/#{sheet}.scss"}
        css_packages[package].each {|file| @output += Sass::Engine.new(File.open(file, 'r').read, sass_options).render}
      end

      response.content_type = 'text/css'
    when :js
      if not js_packages[package].nil?
        js_packages[package].map! {|script| "#{Rails.root}/app/javascripts/#{params[:package]}/#{script}.js"}
        js_packages[package].each {|file| @output += File.open(file, 'r').read}
      end

      response.content_type = 'text/javascript'
    end

    response.headers['Cache-Control'] = "public, max-age=#{1.year.seconds.to_i}" unless Rails.env.development?
    render :text => @output
  end
end

Gemfile

gem 'haml'

routes.rb

match '/stylesheets/:package.css', :to => 'assets#index', :as => 'stylesheet', :type => :css
match '/javascripts/:package.js', :to => 'assets#index', :as => 'javascript', :type => :js