OS X Mavericks Developer Preview 3 has been released for Mac developers. Those running OS X Mavericks Developer Preview 2 can find the new update available in the Mac App Store, accessible via the Software Update menu.
If you are not yet running Mavericks but have a registered developer account, the full installer can be downloaded from the App Store and then found in the Applications directory. The installer app can then be turned into a bootable USB install drive if desired.
OS X Mavericks (10.9) includes over 200 new features and improvements to OS X that many Mac users will be pleased to see, including improved dual monitor support, a tabbed Finder, significant improvements to app power management and battery life of portable Macs, an overhauled Safari, and much more. The full public release of OS X Mavericks is expected to be released this fall.
Apple is speeding relatively quickly through developer releases of their major operating system updates. Earlier today, Apple also released beta 3 of iOS 7 and an update to Xcode 5 Developer Preview.