The autocomplete widget, reintroduced in jQuery UI 1.8, is back and looking better than ever. This is one of my favorite widgets in the library, and although it doesn't yet have the full set of behavior that it had in its first incarnation, it still provides a rich set of functionality to enhance simple text inputs that expect data from a predefined range.
A good example is cities; you have a standard
<input type="text"> on the page, which asks for the visitor's city. When they begin typing in the
<input> element, all of the cities that contain the letter that the visitor has typed are displayed. The range of cities that the visitor can enter is finite and constrained to the country in which the visitor lives ...