With Mountain Lion, the same thing's happening to Lion. All the features of iChat are still there, but the app's been renamed Messages and it now supports iMessage (and is now integrated with FaceTime). You can use Messages to send text or images to anyone on a device capable of using iMessage—namely, devices running iOS 5, and Macs running Messages. Unlike SMS text messages, the iMessage system transfers data via the Internet, so there are no text charges.
Like Messages on the iPhone, Messages for Mac lets you hold multi-person chats and can optionally let people know when you've received and read their messages and when you're typing a reply. An integrated video-chat button allows you to kick off a video chat with capable devices, either over AIM (as iChat has always done) or by launching the FaceTime app.
For iOS 5 users who have been waiting for iChat to support iMessage, this is great news—but having to wait until Mountain Lion's release this summer would be an exercise in frustration. There's good news on that front: Apple says that Lion users will be able to download a beta version of Messages starting Thursday. The final version will be available in Mountain Lion.
For more in-depth information on Messages, check out our Messages beta hands-on.
Enter Notification Center
Sometimes one of your apps needs to get your attention. For years, many Mac app developers have built their own (think reminder pop-ups in iCal or Microsoft Office). The open-source project Growl has for years attempted to create a more general notification system supported by lots of apps.
With Mountain Lion, Mac OS X gains a system-level notification system accessible to every developer, with features much like those already found in iOS. Alerts appear in the top right corner of the screen in a small bubble. Notifications remain there for five seconds, and then slide off screen to the right. Alerts, on the other hand, remain on-screen until you click on the Show or Close (or in the case of some alerts, Snooze) buttons.