Well, someone like .Neo would say : why make one when Growl works?
Sort of hate it when people put words in my mouth, I'm perfectly capable of expressing opinions all on my own... But okay...
In this case I'm glad Mac OS X doesn't have a build-in notification system like Growl. Not because there's a perfectly fine third-party solution - most of the time I prefer Apple's build-in solutions to others -
but because I hate nagware. It's one of the reasons I can't stand Windows and enjoy working with Mac OS X more: It doesn't constantly bug you with what's going on behind the scenes.