Growl is a notification system that plays nicely with the default notifications.
For other alternatives to Apple's Notification Center, see here.
Ranked in these QuestionsQuestion Ranking
Pro Support for a lot more applications than the default notification system
In addition to all the applications that other people have written that support Growl, Growl includes several 'Extras' that serve various functions.
Pro Ability to be themed
Growl comes with almost 20 themes pre-installed so you can pick what works best for you. You can also create and install your own custom themes.
Pro Sync with mobile phone via Prowl
Prowl is the Growl client for iOS. Push to your iOS device notifications from your Mac or Windows computer, or from a multitude of apps and services.