The script.aculo.us has some cool demos of the Ruby on Rails AJAX helpers, which use drag-and-drop and autocompletion features.

What makes Rails great is that with 2 lines of code you get a full application, in our case a cool text input box with autocompetion. Here is the basic example at script.aculo.us:

# view
<%= text_field_with_auto_complete :contact, :name %>

# controller
auto_complete_for :contact, :name


isn't that cool?