A couple of minutes ago, the later CEO of Apple, Tim Cook announced the new major update to Mac OS X series with the launch of Mac OS X 10.8 (Mountain Lion).  Apple has previewed a new version of the Mac OS X called Mountain Lion 10.8; it will be the successor to Mac OS X Lion.  It is now reported that Apple has just seeded the first developer preview of the Mac OS X Mountain Lion and it is now available to download for the users and developers. The first preview version of the Mac OS X Mountain Lion will be seeded to your Mac through Mac App Store and it will replace your existing installation of the Mac OS X Lion or older version of OS X.
Mac OS X Mountain Lion

Before going to install it on your machine, make sure you are not running it on your primary machine as it will screw your existing version of the Mac OS and will replace it with the latest Mac OS X Mountain Lion To download, simply hit your developer account and go to Mac section and check the Mac OS X Mountain Lion Developer Preview or OS X Mountain Lion Server Developer Preview. To install the Mac OS X Mountain Lion on your device, you must be running a 64-bit, Intel-based Mac running either Mac OS X 10.6.7 or OS X Lion and your device must have at least 8 GB free on your device.

In addition to the pre-release version of the operating system, Apple is also making a number of related resources available, including guidelines for developing apps for Mountain Lion, GameKit and GLKit programming guides explaining how to write Game Center-compatible games using social gaming and graphics technologies, which debuted in iOS 5 and more.

Apple has also pushed the first demo video of the Mac OS X Mountain Lion on their YouTube Video channel covering the new features of the Mac OS X Mountain Lion: