What does xmlhttp.php do?
It enables real-time updating of story / comment vote counts when casting a vote, amongst other things.
The following are the functions - the file is easy to understand though no comments are there - just open to understand better:
1] AJAX Vote cast (infact it should be called XMLHTTPREQUEST Voting)
3] field checks in register form (check for availability, continue will get enabled by this)
4] AJAX email
5] AJAX show recommended and e-mail to friends