Previewed at WWDC, launching in the fall.
Apple Seeds First OS X 10.10.1 Yosemite Beta to Developers
The new beta, build 14B17, is available through the Software Update mechanism in the Mac App Store and through the Mac Dev Center.
While it is unclear what improvements the first update to OS X Yosemite will bring, there are several bugs in the operating system including a major Wi-Fi issue that has caused connection problems for a number of users.
Apple is asking developers to focus on Wi-Fi, Exchange accounts in Mail, and the Notification Center.