Apple has seeded the sixth Developer Preview of OS X Mavericks to developers



From MacRumors:

The newest Mavericks preview comes alongside the final release of OS X 10.8.5. OS X Mavericks, which is expected to be released this fall, includes multiple new features like a tabbed Finder, full screen dual monitor capabilities, Safari improvements, and a new Maps app. Developers can receive the update via the Software Update tool in the Mac App Store.

  Apple Releases OS X Mavericks Developer Preview 6