Apple has issued a Golden Master build of OS X 10.9 to all its registered developers. Golden Master meaning this will be (or usually is) the final release. We also get a GM build of Xcode 5.0.1, which is necessary for building OS X 10.9 applications that are to be submitted to the Apple App Store.
OS X 10.9 (also known as Mavericks) has many improvements that power users will appreciate:
New tabbed version of the Finder
Enhancements for multiple-monitor setups more useful.
"App Nap" should also reduce resource usage while increasing battery life.
At this point there are still no concrete release dates but the end of October seems quite likely. Also we would expect new iPads and Haswell Retina MacBook Pros to be on the agenda.