It's already been announced by Microsoft that if you're currently running Windows 7 or Windows 8.1 you'll be able to get Windows 10 as a free upgrade (installed within a year of July 29th). Now Microsoft has announced a new way for anyone to get Windows 10 for free, and it's super easy.
If you install the latest Windows 10 preview (build 10130) on your PC and then use a Microsoft Account that's registered for the Windows Insider program you'll get the final version of Windows 10 free. It will be forever genuine and you can install the preview and upgrade to the final version for free.
Microsoft's Gabe Aul said "As long as you are running an Insider Preview build and connected with the MSA you used to register, you will receive the Windows 10 final release build and remain activated."