- http://drupal.org/project/chosen - see also http://www.lullabot.com/articles/module-monday-chosen
- http://drupal.org/project/autocomplete_deluxe
- http://drupal.org/project/content_taxonomy - provides settings for new term in autocomplete widgets for terms (do not allow new term or specify another vocab for new terms).
- http://drupal.org/project/term_ref_autocomplete - shows hierarchies
Also interesting: An D8 core issue about integration jqueryUI autocompleter instead of the current custom autocompleter: http://drupal.org/node/675446