OS X Mavericks is the most advanced desktop operating system which is becoming this fall. There has been previous releases for developers already and now Apple has released OS X Mavericks Developer Preview 6 which can be downloaded via software update for testing.
This update is recommended for all users running OS X Mavericks Developer Preview 5
Developers can download the Preview 6 update via the Mac App Store. The Mac Dev Center doesn’t list the new preview as of yet but it will very soon.