Preview at WWDC likely in June, followed by September launch.
Apple Releases OS X Yosemite Golden Master Candidate to Developers [Update: Also Public Beta]
The latest update for developers, which arrives as Build 14A379a, can be downloaded from the Mac App Store and through Apple's Mac Dev Center. Apple has also released Xcode 6.1 GM seed and a new version of OS X Server 4.0 developer preview.
OS X Yosemite brings a flatter, more modern look to OS X, with an emphasis on translucency and redesigned dock, windows, and more. It also includes a multitude of new features, such as improved integration with iOS 8 through Continuity, a new "Today" view in Notification Center that offers integration with third-party apps, a retooled Spotlight search with new data sources, and several new features for apps like Mail, Safari, and Messages.
Over the course of the beta testing period, each Developer Preview has added new features and refined the look and performance of OS X Yosemite, although changes have become less obvious in more recent builds as Apple begins finalizing Yosemite ahead of its public release, which is expected in late October.
The current build may or may not be a final version of OS X Yosemite, depending on the outcome of final testing. As an example, Apple last year seeded its planned golden master build to developers on October 3, although it did release a second golden master version a few weeks later just ahead of the public release on October 22.
Update 11:07 AM: Apple has also released the fourth public beta of OS X Yosemite, which carries a slightly different build number of 14A379b compared to the golden master candidate build distributed to developers.
Update 11:54 AM: Both the developer and public beta releases include the fix for the "Shellshock" bash security flaw. Apple released fixes for OS X Mavericks, Mountain Lion, and Lion yesterday.