How does StackOverflow's 'tags' textbox autocomplete work?

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.

From stackoverflow

• Autocomplete. That's the one. Possibly not the exact version used on SO, but it'll do the job.

  See also: What’s a good AJAX Autocomplete Plugin for jQuery?

  Erik Forbes : Awesome, thanks a bunch - this is exactly what I was looking for. =)