Alongside the release of iOS Beta 5, Apple also released a new preview of their OS X Mavericks OS, which is the tenth operating system to be released for the Mac, and brings a few new features that were announced during WWDC 2013.
Currently only available for developers to download, Preview 5 in particular brings a new beta version of iPhoto, a new iBooks app and a few new bug fixes and improvements.
Apart from that, there’s not much else we know about so far.
To find out more developers can download the update via the Software Update tool in the Mac App Store, or download the software from the Apple Developer Portal.
Source: Mac Rumours