StackOverflow的“标签”文本框如何自动完成?

I know they're using a jQuery plugin, but I can't seem to find which one they used. In particular, what I'm looking for is autocomplete with exactly the same functionality as SO's autocomplete, where it will perform an AJAX command with each new word typed in and allow you to select one from a dropdown.I know they're using a jQuery plugin, but I can