The macosxhints Rating:

[Score: 9 out of 10]
So why would you want to be notified about events? I'll give you a couple examples of how I use Growl. I use the GrowlMail extra (in the Extras folder on the Growl disk image) to pop up a small message showing the sender, account, and subject of the message when I receive a new email in Mail, as see in the image at left. If I click in the window, that message will open in Mail.
I use the Transmit notifiers all the time, so that I'll know when a large upload is complete. The recent PotW selection HandBrake includes a Growl notification for "Encode Done," so I don't have to keep checking the app to see its progress. There are other apps I use with Growl, but you get the idea -- Growl proactively tells you when whatever it is you're waiting for has happened. Growl is also a popular subject here on macosxhints.com, with 20 Growl-related tips in the system.
You can configure Growl notifications in any number of ways, changing their appearance, style, and duration on an application- and event-specific basis. If you don't like any of the built-in notification styles, you can download some others, or even (if you have some HTML and CSS abilities) create your own. To do its work, Growl uses an always-running background process (GrowlHelperApp), but I've not noticed any problems from this process.
Growl isn't for everyone, but if you prefer the system telling you about things when they're done instead of having to actively check them for yourself, it's well worth a look.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070926092609653