A simple class that enables you to directly download a video from YouTube by simply providing the full YouTube URL.

YouTube.rb

require 'rubygems'
require 'json'
require 'open-uri'
require 'progressbar'

# YouTube
# This class allows you to directly download videos straight from YouTube
# All you have to do is provide the full URL to a video on YouTube.com
# See the examples below!
class YouTube
  
  BASE_URI = "http://www.youtube.com/"
  
  def self.read_video_from(url, &block)
    download_video(parse_video_url(url), &block)
  end
  
  def self.read_video_from_and_download_to(url, path = Dir.pwd)
    full_path = File.expand_path(File.join(path, parse_video_title(url) + '.flv'))
    read_video_from(url) do |youtube|
      File.open(full_path, "w") do |video|
        video.write(youtube.read)
      end
    end
  end
  
  def self.parse_video_title(url)
    open(url) do |file|
      file.each_line do |line|
        return $1 if line =~ /<h1 >(.*)<\/h1>/
      end
    end
    false
  end
  
  private
 
    def self.parse_video_url(url)
      url       =~ /watch\?v=(.*)/ 
      video_id  = $1.split("&")[0]
      video_url = nil
      open("#{BASE_URI}watch\?v=#{video_id}") do |page|
        page.each_line do |line|
          if line =~ /.SWF_ARGS.:(.*),\n$/
            params    = JSON.parse($1)
            video_url = "#{BASE_URI}get_video?video_id=#{params['video_id']}&t=#{params['t']}"
            break
          end
        end
      end
      video_url || could_not_locate_video_file
    end
    
    def self.could_not_locate_video_file
      puts "Could not locate video file for the given URL!"
      exit
    end
 
    def self.download_video(url, &block)
      puts "Downloading Video From YouTube..."
      progress_bar = nil
      open(url, :proxy => nil, :content_length_proc => lambda { |length|
        if length && 0 < length
          progress_bar = ProgressBar.new(url, length)
        end 
      },
      :progress_proc => lambda { |progress|
        progress_bar.set(progress) if progress_bar
      }, &block)
      puts ""
      puts "Finished downloading video!"
    end
    
end


## EXAMPLES

## URL
# All you need is "a very awesome video"
url = 'http://www.youtube.com/watch?v=oHg5SJYRHA0'

## METHOD 1
# Then you can simply invoke the following "One Line Command" to download
# and directly save the video to the filesystem
YouTube::read_video_from_and_download_to(url, 'youtube')

## METHOD 2
# If you wish to do something else before writing the YouTube video you can
# also use the following method to read from YouTube and then later store it on the filesystem
# To manually fetch the "video title" from YouTube, use: YouTube::parse_video_title(url)
# be sure to append the ".flv" extension to it, as it of course isn't stored in the YouTube video title
YouTube::read_video_from(url) do |youtube|
  File.open("/Users/Michael/Desktop/#{YouTube::parse_video_title(url)}.flv", "w") do |video|
    video.write(youtube.read)
  end
end

## METHOD 3
# Basically like method 2, except that we're not using the YouTube::parse_video_title(url) method here
# which means you have 100% control over what the output file must be called.
YouTube::read_video_from(url) do |youtube|
  File.open("/Users/Michael/Desktop/my_video.flv", "w") do |video|
    video.write(youtube.read)
  end
end