Apple has released OS X 10.9 Mavericks Developer Preview 6 to developers, with minor improvements.
OS X Mavericks is set to arrive in the Mac App Store this autumn. See: Mavericks release date
OS X Mavericks has new features, a new look, and a new theme (no more cats). New features include better support for multiple monitors, Finder tabs, improvements to performance and battery consumption, new Maps and iBooks apps and updates to Safari and Calendar, each new beta version brings more details and new improvements to the upcoming operating system.
Apple is believed to be planning a 10 September special event, which could see the launch of two new iPhones, and could also shed some light on when we can expect Mavericks to be released.