Build 12D61 has been released less than a week after build 12D58. The new build asks developers and testers to focus on AirPlay, AirPort, Game Center, Graphics Drivers, and Safari.
As with previous builds, Apple mentions that it has significantly improved WiFi performance, as noted by 9to5Mac. Registered developers can download the update on Apple's Developer Page.
This post originally referred to Build 12D61 as the ninth developer seed of OS X 10.8.3 when it was actually the eighth seed.