I'll be releasing a robust private messaging module very soon. Features include:
* Threads, messages, users
* Multiple users/messages per thread
* Message creation with live search for adding/removing recipients
* Message search
* Show all, unread, sent, deleted
* Mark as unread, read, deleted
* Email notification
* Live notification for active sessions
* Global functions decorated into SiteTree including: $NewMessagesCount and $SendMessageLink for access to module features on any page
* Loaded with AJAX
Wow! That looks much more mature than the one I chose. Too bad the demo doesn't work! :(
I'm delaying the release to give me a chance to clear my plate so I'll be able to handle writing documentation and supporting it, etc. The genesis of the module is that it's a component to a social networking site I've been working on since the beginning of the year, so once that goes into beta, the module will be well-tested and I'll put it up here. Still a little buggy.
Oh wow, that demo was working when I posted that comment, must have gone down right after!
Like I said though, I was looking to make a pretty similar field (although mine would have been lackluster in comparison to yours I'm sure...). If there is any way I could help you out and possibly get a copy of that field pre-release, let me know! :)