download_youtube_videos.rb

require 'rubygems'
require 'mechanize'
require 'nokogiri'
require 'open-uri'

def build_url(html)
  title_begin = html =~ /<title>/
  title_end = html =~ /<\/title>/
  
  # get the title - ignore the YouTube prefix
  title = html[title_begin + 17, (title_end - title_begin) - 17]
  
  # need this "t" param based on bookmartlet from 
  # http://googlesystem.blogspot.com/2008/04/download-youtube-videos-as-mp4-files.html
  t = html[(html =~ /"t"/) + 6, 44]
  video_id = html[(html =~ /"video_id":/) + 13, 11]
  is_hd_available = html[(html =~ /isHDAvailable/) + 16, 5]
  format = is_hd_available =~ /true/ ? "22" : "18"
  
  url = "http://www.youtube.com/get_video?fmt=#{format}" +
        "&video_id=#{video_id}" +
        "&t=#{t}"
        
  { :title => title.gsub("/", "-").gsub(":", "-"), :url => url }
end

agent = WWW::Mechanize.new

# youtube_video_ids just contains the ids (the part of the url after the "v=" 
# when watching in your browser) one per line
ids = File.new("youtube_video_ids.txt", 'r')
ids.readlines.each do |id|
  video_url = "http://www.youtube.com/watch?v=#{id.chomp}"
  html = Nokogiri::HTML(open(video_url))
  video = build_url(html.to_s)
  page = agent.get("#{video[:url]}")
  File.new("/path/to/downloads/#{video[:title]}.mp4", "w").write page.body
end