Example (updated for Rails 3):

  # config/routes.rb
  ..
  post "/blog/auto_complete_for_post_title" => "blog#auto_complete_for_post_title"
  ..

  # Controller
  class BlogController < ApplicationController
    auto_complete_for :post, :title
  end

  # View
  <%= text_field_with_auto_complete :post, :title %>

  # stylesheets 
  (this is not necesary or even the best fit, but a quick working solution if the boxes doesn't fit nicely below the auto complete textbox)
  * { margin:0; padding:0; }

By default, auto_complete_for limits the results to 10 entries,
and sorts by the given field.

auto_complete_for takes a third parameter, an options hash to
the find method used to search for the records:

  auto_complete_for :post, :title, :limit => 15, :order => 'created_at DESC'

For more examples, see script.aculo.us:
* http://script.aculo.us/demos/ajax/autocompleter
* http://script.aculo.us/demos/ajax/autocompleter_customized

Copyright (c) 2007 David Heinemeier Hansson, released under the MIT license