by 
OS X 10.9 Mavericks Developer Preview 1 is now available to download directly from Apple. Registered developers can access the installation from Apple’s Developer Center. Much like prior OS X betas, installation of OS X Mavericks is done through the Mac App Store, and it is highly recommended to back up OS X with Time Machine before beginning installation of any beta operating system.
Get the first developer preview directly from Apple
It appears that most Macs capable of running OS X 10.8 are to be eligible to run 10.9, though many of the iOS-to-OS X features will rely on iOS 7 or newer. To test those cross platform features, developers registered with Apple will also find iOS 7 beta 1 is available as well.
OS X 10.9 includes over 200 new features and enhancements, including Maps, iBooks, a revised Notification center with shared alerts from iOS devices, an all new Safari, an iCloud password keychain, tabs and tags in the Finder, multi-display support with full screen mode, and a wide variety of other improvements under the hood. Much of the refinements in OS X 10.9 are aimed at improving performance of Macs, including extending the battery life of portable devices.
OS X Mavericks is scheduled for a public release this fall.
