Here I've got an example of it at:
http://www.flash-db.com/index.phpTells you who's online, user's, post's, registered members. Can be used on any page. Look on the Top part of that page - you won't miss it. You need Flash 5 installed though.
The example for it is in Flash. But it's the same concept when using HTML.
I don't know how I would go about writing a Mod for it though. I'm currently adding some new parts as well such as the Last Post etc - that can be added to any page. Couple other things as well.
I've actually just been writing my own code for it based on the Yabb database table structures.
my Board Example:
http://www.flash-db.com/Board/