Apple Seeds OS X Mavericks 10.9.5 Build 13F12 to Developers
The beta is available through the Software Update mechanism in the Mac App Store and through the Mac Developer Center.
It is unclear what improvements the 10.9.5 update will bring to Mavericks, but it is likely to include bug fixes and stability enhancements. Apple is asking developers to focus on USB, USB Smart Cards, Graphics, Safari, and Thunderbolt.
Along with working on improvements to Mavericks, Apple is also beta testing OS X Yosemite, which is due to be released in the fall. The last Mavericks update, 10.9.4, added several Wi-Fi fixes and improved wake from sleep reliability.