This is theory at the moment, but could be a very nice standard way to show downloads, itunes status and new mail among others…
For users, this means that all your apps, if Growl enabled, will display pretty notifications in the same, uniform way. You can also use growl to work with any scripts you write in perl, python, and other things. You can also decide what applications may, or may not, send you a notification, and then the types of notifications it can send you. [Growl!]