foo.rb

class DownloadManager
  def initialize
    @downloads = []
  end

  def save(url, filename)
    downloads << Lazy.future { File.binwrite(filename, open(url).read) }
  end

  def finish_all_downloads
    downloads.each { |d| Lazy.demand(d) }
  end

  private

  attr_reader :downloads
end

downloader = DownloadManager.new 

downloader.save("http://prawn.majesticseacreature.com/manual.pdf", "manual.pdf")
puts "Hey now, that was fast!"

downloader.save("http://sandal.github.com/rbp-book/pdfs/rbp_1-0.pdf", "rbp_1-0.pdf")
puts "So was that!"