Microsoft is expected to release a preview version of Windows 9 (code-name Windows Threshold) by late September or early October, according to ZDNet. For those who don't know, Windows Threshold will include a number of new features that are aimed at continuing to improve Windows' usability on non-touch devices and by those using mice and keyboards alongside touch. A new Start Menu, windowed Metro apps that can run on the Desktop, virtual desktops, and the elimination of the Charms are some of the new features. Microsoft is also making several UI changes to Windows 9.
A second preview build can also be introduced in early 2015, where as the final version can be expected in the spring of 2015.