When you're working on your Mac, sometimes things happen in the background that you would like to keep track of while working in another application. Growl works with hundreds of popular Mac apps to notify you of specific events, like when a download is completed or when your CD-burning project is finished.
For example, it can notify you when a friend comes online in Adium, when your EOD backup is done, and even pop up abbreviated versions of incoming e-mails--all while you're working on something entirely different, often with the help of app-specific plug-ins like GrowlMail, GrowlTunes, and GrowlSafari.
We like having control over what gets a Growl notification (which you can handle through easy preference pane settings) and the ability to choose from numerous skins to customize the look of each notification. Overall, if you're looking to always know what's going on with your running applications, this free program is an excellent way to stay notified. The newest version requires Mac OS X 10.5 or later.