We knew these were coming to the next version of OS X already, but Apple has just demoed all three of them at the WWDC Keynote in San Francisco today. As you might expect, all three of these famous iOS features look pretty much the same on a Mac (especially now that the sweet MacBook Pro with Retina Display has been announced).
Notifications allows you to hear about things like Twitter direct messages and other pending notes from a dragdown menu on the top of your screen, AirPlay allows you to send video wirelessly off to other devices, and Game Center not only tracks achievements, but allows for multiplayer gameplay across other computers and the Internet.
All of these look great, and it looks like Apple has been able to bring all of the functionality over (except, of course, for the actual touchscreen). Mountain Lion should be out later on next month, with more info to come. Stay tuned.