Apple is seeding a second preview of OS X Mavericks
to developers. The release is available through the Mac App Store, and listed as build 13A497d. Some known issues are in the code, including display problems with Safari and the new iCloud Keychain feature.
Mavericks makes a number of feature improvements to OS X, for instance adding native Maps and iBooks apps, and a tabbed version of Finder. Many changes though are technical, such as App Nap and Compressed Memory, simply making more efficient use of memory, power, and processors. The finished Mavericks release is due in the fall.