This fork works for 'tags', not 'categories'.

jekyll-create-tag-pages-rakefile.rb

desc 'Generate tags page'
task :tags do
  puts "Generating tags..."
  require 'rubygems'
  require 'jekyll'
  include Jekyll::Filters
  
  options = Jekyll.configuration({})
  site = Jekyll::Site.new(options)
  site.read_posts('')
  site.tags.sort.each do |tag, posts|
    html = ''
    html << <<-HTML
---
layout: main
title: Posts tagged "#{tag}"
---
    <h1 id="#{tag}">Postings tagged "#{tag}"</h1>
    HTML

    posts.reverse.each do |post|
      post_data = post.to_liquid
      html << <<-HTML
        <div class="section list">
        <h1>#{date_to_string(post.date)}</h1>
        <p class="line">
          <a class="title" href="#{post.url}">#{post_data['title']}</a>
        </p>
        <p class="excerpt">
	  #{post_data['excerpt']}
        </p>
        </div>
      HTML
    end
    
    File.open("tags/#{tag}.html", 'w+') do |file|
      file.puts html
    end
  end
  puts 'Done.'
end