Example of using http_request to only download files if the mtime changes.

recipe.rb

remote = "http://couchdb.apache.org/img/sketch.png"
local  = "/tmp/couch.png"

remote_file local do
  source remote
  action :nothing
end

http_request "HEAD #{remote}" do
  message ""
  url remote
  action :head
  if File.exists?(local)
    headers "If-Modified-Since" => File.mtime(local).httpdate
  end
  notifies :create, resources(:remote_file => local), :immediately
end