Not ready for primetime yet!

http://github.com/redinger/auto_complete
https://svn.nbarthelemy.com/plugins/local_auto_complete
http://themysteriouswaysofruby.blogspot.com/2007/04/fast-autocompletion-with-rails.html

Example:

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

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

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