What's all this, then?
So you have Google Notifier and you're unhappy with that "new message pop-up" window announcing your new mail or calendar events. How do you change it? Simple: you install Google+Growl (a plugin for Google Notifier).
Growl notifications are small windows showing some text. Out of the box, Google Notifier shows its own pop-up and doesn't sends out those notifications, but Google+Growl helps Google Notifier send Growl notifications instead. Because you can pick between a variety of Growl displays, your notifications can look pretty much however you'd like. And Growl notifications are sent out by other applications as well, so everything can look consistent.
Why does this only run on Mac OS X v10.5 ("Leopard")?
Due to an implementation detail in Mac OS X corrected in a certain way by Apple, releasing Google+Growl in a version compatible with Mac OS X v10.4 would mean that you couldn't edit the notification texts the way you'd like to. (Specifically, some spaces would be removed.) We're investigating a workaround to this.
Why doesn't clicking the notification take me to the correct link if I'm using Gmail/Google Apps for my own domain?
This is a known bug in Google Notifier (Google+Growl simply isn't sent the correct address) and it has been reported. The bug has been fixed and will be available shortly. For more information, please contact Google directly.