A Growl style that mimics the alerts in Mountain Lion's Notification Center.
Download the Zip file, unzip and rename to MountainLion.growlStyle . Then you can double-click to let Growl handle the installation.
git clone git://github.com/erkreutzer/MountainLion.growlStyle.git ~/Library/Application\ Support/Growl/Plugins/MountainLion.growlStyle