There are plenty:
http://code.drewwilson.com/entry/autosuggest-jquery-plugin (broken link github link)
- http://bassistance.de/jquery-plugins/jquery-plugin-autocomplete/
http://plugins.jquery.com/project/tag-it (broken link github link)
http://plugins.jquery.com/project/jquerytag (broken link)
If you are looking for a styled and ready solution, I'd recommend the first option.
If you are looking for a more flexible and complete solution, the second solution is probably a good fit.