Previewed at WWDC, launching in the fall.
Apple Seeds First OS X 10.10.5 Yosemite Beta to Developers
The new beta, build number 14F6a, can be downloaded from the Apple Developer Center.
It is not yet clear what's new in the OS X 10.10.5 beta, but like 10.10.4 and earlier betas, it likely includes bug fixes and performance enhancements.
OS X 10.10.4, the current version of OS X Yosemite, introduced multiple bug fixes and under-the-hood performance improvements, including the removal of the problematic Discoveryd process.