[Toolbox] [Example] [Source]
A common practice to simulate a Windows "Combo Box" is to have a text box where a user can type a value, plus a select box which holds valid options. If the user selects an option from the select box, it gets filled into the text box.
This script allows you add better presentation to this concept by completing the user's entry as they type into the text box. As it matches options available in the select list, the option gets selected and (in IE) the text-entry box is auto-completed and highlighted. This works in a way similar to many Windows programs.