Growl, perhaps the most well-known and popular notification system for OS X, has just released version 1.3 as a Mac App Store exclusive. The previously free notification system has become a paid app with Growl 1.3. As Macstories points out, the reason the Growl team decided to make what was previously a free System Preferences pane into a US$1.99 app was so users would be provided with an up-to-date app utilizing the latest Apple technologies.
So what's new with Growl 1.3? Almost everything. Growl has been rewritten from the ground up using a new architecture, sporting a new UI, and adding lots of features previous iterations of Growl never had. But not to worry, Growl 1.3 still retains everything you loved about Growl 1.2, they've just added a lot of cool new features to it.
Among the coolest new features is a Notification Center-like Rollup window. The window, which is accessed from the Finder's menu bar (where you find Growl now), keeps a history of all your notifications that you've received from various apps that use Growl. This is especially handy when you're away from your computer and want to see what notifications you missed.