I finally got around to making a re-usable AJAX auto-suggest field. Here it is in action.

The AutoSuggest class adds a popdown menu of suggested values to a text field. The user can either click directly on a suggestion to enter it into the field, or navigate the list using the up and down arrow keys, selecting a value using the tab key. The values for the suggestion list are to provided as XML (by a PHP script, or similar).