dhtmlxCombo - Ajax Autocomplete

Easy configurable DHTML JavaScript combo box with Ajax support, autocomplete functionality and wide cross-browser compatibility. Features include type-ahead, keyboard support and ability to display images in the dropdown list. dhtmlxCombo can be easily converted from existing HTML SELECT or populated with JavaScript. The component works fast with large sets of data, loading list of suggestions dynamically on demand



Read more and download the source from the below link.
Demo is also available at the same link.

http://dhtmlx.com/docs/products/dhtmlxCombo/index.shtml