Apple have just released the latest beta in the OS X Mavericks family. Build 13E16 is now available to download from the OS X developers portal or via the built in Software Update facility.
This particular build is said to be focused around improvements to the stock graphics, Bluetooth and Audio drivers, Finder and Safari.
Developers have been asked to focus their testing on the refreshed areas, paying specific attention to website compatibility in Safari 6.1.5.
As always, its best not to install these releases on your day to day work horse.