Hi! I would suggest a new Tag selection type that behaves exactly like Drupal's freetagging vocabulary and suggested terms:
http://drupal.org/project/suggestedtermsI've noticed that there is a huge memory issue with Custom Field types such as: "Alphanumeric > Multiple select", radio buttons, or checkboxes when there are lots of items you could select. IMHO, an AJAX input field like in freetagging would be a great improvement...
Maybe we could implement Drupal's existing freetagging and vocabulary system into CiviCRM?
http://zivtech.com/blog/howto-add-a-freetagging-form-a-node-display-drupal-5