Growl 2.0 is now integrated with Notification Center in Mac OS X Mountain Lion. The new update means they should start playing nice with each other and stop being a pain in the backside for users.
Up until now, it has been a giant pain to use both, since banner alerts from Growl and banner alerts from Notification Center would show up in the same spot by default. So you would either have to ditch Growl, move the location of its notifications, only use apps that support Growl, or use some kind of third-party hack. Fortunately, an official update from Growl's developers means life with Mountain Lion is less annoying. [Growl via Cult of Mac]