Apple has released yet another seed for OS X 10.8.3 today, Build 12D50, to developers in the Mac Dev Center. This is the fourth seed since the original 10.8.3 beta back in November.
The developer notes state that the focus areas for this beta seed are AirPlay, AirPort, Game Center, Graphics Drivers, and Safari. After developers use the new seed with their applications, they typically file bug reports with Apple, who can then make changes and fix any issues that may still be in the OS X operating system.
If you have a developer account and want to see what’s new in 10.8.3, you can log in and download the beta seed right now.
Source: Apple Developer Center