Why use Growl?

It works out of the box.
When you start up a Growl-enabled application, it will just work. There's no complicated setup; just install Growl once and your apps can start displaying notifications right away.
Everything in one place.
Growl centralizes all your notification preferences into its preferences - you can control them all from one place, and you know exactly how they are going to behave.
Know what happened while you were away.
Keeps working even when you are not. Come back to your mac and review what happened while you were gone with Rollup. Plus get notifications on the go on your iPhone or iPad via Prowl
Support for themes.
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.
Complete control.
Growl offers you complete control over which notifications are shown and how they are displayed. You can easily turn notifications (specific ones or all of them) off.
Easy to support for developers.
If you're interested in supporting Growl in your application or working on Growl itself, check out the developer docs and download the Growl SDK.