How does PM recipient field auto populate?

[Bilderback]

When sending a new PM, as you type in the recipient field, usernames appear in a dropdown
menu as you are typing.
Does anyone know how this is accomplished?

I think it would be a great thing to add to other field areas, like 'Add Moderator'
and custom forms.
Thanks

[Dismounted]

AJAX - have a look at the HTML source of that page.