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As with the Twitter integration in iOS 5, Mountain Lion now has Twitter built in to your Mac. Sign in once to tweet from your apps, receive Twitter notifications, and view your Twitter friends' photos and usernames in Contacts.
Game Center for Mac
The popular social gaming network from iOS will now be on the Mac with Mountain Lion. See what your friends are playing, check the leaderboards, or discover new gaming friends that Game Center recommends for you. The Game Kit for developers even offers tools for creating multiplayer games that can be played on both Macs and iOS devices.
Gatekeeper protects your Mac
A major new Mac security feature, Gatekeeper helps you keep your Mac safe from malware with options to only download software from Apple-approved developers and third-party apps with an Apple Developer ID.