Previously announced by its developers, a major update to Growl – a third-party notification system for OS X – has been released today with official Notification Center integration. Among various bug fixes, improvements to the display of some built-in themes, and separation of Action displays from Visual ones, Growl 2.0 also brings Prowl and Boxcar support.

The biggest news in this update is support for Notification Center on Mountain Lion. How it works is rather straightforward, too. If users decide they want to keep using Growl while consolidating the Visual displays in Apple’s Notification Center, they can open Growl’s Preferences, and set “OS X Notifications” to “On” under General. This will add Growl to the apps listed under Notifications in System Preferences, and it’ll effectively use Growl as a bridge between third-party apps and Notification Center.

In my tests, it worked reliably: as soon as an app with Growl support triggered a notification, that was forwarded to Notification Center immediately. The obvious downside is that, because of Apple’s restrictions, Growl won’t be able to apply its custom themes to Notification Center, or use third-party app icons for banners. So every Growl notification will carry Growl’s icon, not the one from the third-party app that triggered the original notification.

Update: For apps to show their own icons, they will need to support the Growl 2.0 SDK.

Another important addition to Growl 2.0 is the separate handling of Visual and Action displays. For instance, you can set, say, Dropbox to trigger a Smoke display (or Notification Center banner) and the MailMe or Prowl action at the same time. This is a powerful new feature that should allow for deeper customization of Growl notifications, which are handled on an app-by-app basis in Growl’s Applications tab.

Developers of Growl-enabled apps can find several technical details over the the official Growl blog. Growl 2.0 is available on the Mac App Store for $3.99.