BuddyPress.org

Because this is the thread about it I found 2 more that were not mentioned:

http://www.123flashchat.com/web-messenger-chat.html ( i dont know if it integrates with buddypress)

http://www.gixawchat.com/

and this developer project:

Gmail/Facebook Style jQuery Chat

123flashchat looks interesting it says:

<script language="javascript">

var username_123webmessenger=<?php echo $_SESSION['user'];?>

</script>

to integrate it uhm have to lookup for buddypress/wp what the current logged in username is.