it looks rather like this:
The notifications aren't clickable, not to mention customizable (anyone ever tried changing the 'notification-properties'
despite that, if i'm not around my laptop/desktop, i won't be able to see what i missed.. (unlike in Kubuntu). Xubuntu has a new notification system as well with the same ideal.
On the other hand, there's Growl - An Open Source notification system for Mac OSX - http://growl.info/
It looks much more polished, efficient, and usable (disclaimer: i never tested it nor used OSX)
and yes - it's Open Source. BSD licensed.
i understand that there was an attempt to do such a project back in Jaunty (9.04) - http://colinharrington.net/blog/2009/05/grails-growl-like-notifications-in-linux-ubuntu-904/
and i would be glad to see this rise again..