Apple’s major operating system update, OS X 10.8 Mountain Lion, is now available for download from the Mac App Store for $19.99. The update adds a list of new features and improvements, including AirPlay for beaming your content to an Apple TV, Messages for sending and receiving iMessages on your Mac, Notification Center, support for speech-to-text dictation, and software updates during sleep with PowerNap. Also, on the security side, Gatekeeper brings Apple-signed digital certificates to apps that aren’t sold (or can’t be sold) on the Mac App Store.

The news comes just over two weeks after the company released its Golden Master build to developers. Unlike Lion, Mountain Lion is App Store only (Apple isn't selling USB keys with the software this time around), so you’ll need to be running Lion to upgrade. Lastly, don’t forget — if you bought a new Mac on or after June 11th you’re entitled to a free copy of Mountain Lion through Apple’s Up-to-Date program.

Check out our Mountain Lion review.