I noticed an undocumented feature with the AutoComplete control of the awesome script.aculo.us JS-library.

To display an activity indicator while the control is building up the list of items matching the input, one can add the option “indicator” together with the ID of the element which is to be displayed during the lookup:

new Ajax.Autocompleter('autocomplete'
, 'autocomplete_choices'
, '{$url}'
,{ indicator:'busy' });

This enhances the usability of the AutoComplete control, since with this option, the users knows when the lookup is finished, which is especially important when there are no matching items found.