Growl works great for system notifications
Mac OS X is so easy to use that, before you know it, you’re running many applications simultaneously to perform different tasks, like uploading to your web site with Transmit and downloading the morning news with NetNewsWire. And you’re probably surfing the web and checking e-mail at the same time.
But, with Transmit buried in the background beneath your browser windows, how do you know when that upload finishes? And is NetNewsWire still downloading? Sure, those apps change their Dock icon, but that doesn’t help you much if you usually hide your Dock.
There’s a better way: try Growl.
Now, when that upload completes, you’ll see an attractive notification telling you the file upload is done. The notification includes the Transmit icon, but it’s really Growl conveying the notification from Transmit to you.

And when NetNewsWire finishes downloading your morning news, here’s what you’ll see:

Growl is a system-wide notification system. Those notifications appear because Transmit and NetNewsWire support Growl. The developers modified their applications to check for Growl when interesting events like completed downloads occur and, when they do, the applications tell Growl to tell you.
If you’re wondering, why doesn’t the application just tell me directly?, the answer is that Growl makes notifications consistent across applications—and you control how those notifications behave and appear. By default, Growls displays notifications like the ones featured above using one of a wide variety of display styles (the one shown here is called “Smoke”), but you can also choose to be notified by e-mail or by voice.
Growl’s flexibility is particularly nice. You can specify your preferences at the system level, the application level, and the event level. Here’s the preferences window for Transmit’s Did complete download event:

There’s a lot more to Growl, so check it out!