Growl recently roared its way onto the Mac App Store. This historically free utility now costs $1.99, but we think that it's such a small price to pay for such a wonderful utility -- especially one that has been used day-in and day-out by most all Mac users over the years.
Growl provides pop-up notifications for various applications on your Mac desktop. For instance, if you were to receive a direct message in the Twitter for Mac application, you could set a Growl notification to alert you. Notifications can be themed, and can take on many different shapes, sizes, and styles.