Auto complete solution, with customizable results (return just the word or word+id…) and no framework dependency, just use a plain text_field + class=> autocomplete and every autocomplete libary you like (included: jQuery).

# Controller
class UserController < ApplicationController
  autocomplete_for :user, :name
end

#Customized....
autocomplete_for :user, :name do |items,method|
  render :text => items.map{|item| "#{item.send(method)} -- #{item.id}"}.join("\n")
end

# View
<%= f.text_field :auto_user_name, :class => 'autocomplete', 'autocomplete_url'=>autocomplete_for_user_name_users_path %>

# Routes
map.resources :users, :collection => { :autocomplete_for_user_name => :get}

#JS
#any library you like
#(includes examples for jquery jquery.js + jquery.autocomplete.js + jquery.autocomplete.css )
jQuery(function($){//on document ready
  //autocomplete
  $('input.autocomplete').each(function(){
    var input = $(this);
    input.autocomplete(input.attr('autocomplete_url'));
  });
});

#Model(input/output association)
class User
  find_by_autocomplete('name')
end
class Post
  autocomplete_for('user','name') # auto_user_name= + auto_user_name
end
.

Not as thought free as the default version, but gives you a lot more control.

script/plugin install git://github.com/grosser/simple_auto_complete.git
README