Previewed at WWDC, launching in the fall.
Apple Seeds First OS X Mavericks 10.9.3 Beta to Developers
Apple is currently asking developers to focus on Graphics Drivers and Audio, and there is no word yet on specific fixes and changes included in the update.
The last Mavericks update, OS X 10.9.2, added FaceTime Audio, the ability to block iMessage users on the Mac, and a fix for a critical SSL vulnerability. 10.9.2 saw a beta testing period of approximately two months, and it is likely 10.9.3 will also see several beta iterations before a public release.