Notifications Layer WPMU Dev blog style - how did you do it?

The little bell icon with the number of notifications waiting that pops open a list of the latest WPMU Dev blog posts is both intuitive and brilliant! Was this created using a plug-in?

This would be an incredibly useful way of alerting site members of the latest content. I would love to be able to incorporate the functionality into my own blog.

Thanks so very much for any tips folks might be able to provide on this!
Susan.