Windows 10

There's been talk about Windows 10 and by the looks of it it'll be out this year (2015):

Public release:

Windows 10 is expected to be released to manufacturing in mid-2015. While previous reports suggested a late-2015 release with an RTM in August (in line with past versions of Windows), it was reported that Microsoft wanted to ensure that devices running the operating system would be released in time for the Back to school shopping season in North America and the United Kingdom.

Users will be able to upgrade to Windows 10 using either an ISO image, or through Windows Update. Windows 7 SP1 and Windows 8.1 with prerequisite updates will support a direct upgrade path to 10 via Windows Update. Those using Windows 7 RTM, 8, or 8.1 without the prerequisite updates must upgrade using the ISO. A separate update with "some" of the changes of Windows 10 is planned for Windows RT; as such, Microsoft does not officially consider these devices to be compatible with Windows 10.

On February 2, 2015, Microsoft announced that it would provide a free ARM port of Windows 10 for Raspberry Pi 2 devices through its Internet of Things developer program.