According to MacWorld's article on OSX (Apple's latest version of the Mac operating system, due out later this year), they're finally adding in a notification system to the operating system.
The inability for apps to post simple status messages to their users (in the top or bottom corner of the screen) has always proved a gaping hole in the OS, but one which was filled by the open source Growl project. Lots of apps use it to notify their users when important things happen (like when people log in and out of Skype, or an upload is finished) and although it doesn't come with a new Mac (which creates confusion for new users who are used to Windows' balloon notifications), it's available on the App Store and has always been free (as far as I know).
But as software firms often do, Apple have plugged the gap with their own solution, rather than using what people are already familiar with, and integrating it into their OS (because that would mean Apple admitting they'd missed the boat). In fairness to Apple, their system goes a lot deeper than Growl, and is inspired (read: lifted) from iOS, which in turn was inspired (see above) by Android. MacWorld have more details about Notification Centre.
I imagine most OSX developers will start implementing native notifications quite quickly, but it will undoubtedly create a level of confusion between the two systems. Will all developers be savvy enough to send updates to one system or the other (and not both at the same time)? Will some developers choose to stick with Growl? What about apps which aren't actively maintained anymore?
There are lots more features coming in Mountain Lion, but I just wanted to highlight Notification Centre as I often see stories like this (see Twitter releasing the Tweet button and kicking TweetMeme out of the game), and it's never good to see.
Having said that, I am of course looking forward to playing with the new OS. I am still a fanboy... for now :)