Microsoft has announced that Windows 10 will be available on July 29. It will be a free upgrade for users of Windows 7 and Windows 8. They've also announced that if you're in the free Windows Insider program, you will get a copy of Windows for free as well. You can still get into the program at insider.windows.com. The Windows Insider program is Microsoft's way of testing Windows 10 on the desktop and Windows Mobile 10 for the smartphone.
The upshot of this is that Microsoft is giving away Windows 10 to anyone who wants it. It's possible that as long as you're using an Insider Preview Build of Windows 10 with a Microsoft Services Account, you'll be able to upgrade to Windows 10 for free.