After looking for a jQuery plugin for handling a 'tag suggestion' form field, in much the same way ZenDesk.com does, I ended up developing a customization of jQuery UI that does the same interaction. Check the example.html for a demo. Notice, to make it work under IE you have to add 'js/ie-compat.js' into your pages.
amitsnyderman/tag-it
A jQuery UI plugin to handle multi-tag fields and also tag suggestions/autocomplete.
JavaScript