Who Gets A Free Upgrade To Windows 10?

Microsoft has clarified the "free upgrade" situation of it's forthcoming Windows 10 operating system, which is currently undergoing public testing. Essentially, if you are a home user with a legitimate, licensed current version of Windows  7, Windows 8 or Windows 8.1 you can upgrade for free for one year after the launch date of Windows 10 (likely some time next Summer). Microsoft may even be a little squishy on the "one year" thing. "Current version" means, for example, that Windows 7 has Service Pack 1 installed and is otherwise up to date.

However, if you have a janky, unlicensed version of Windows 7 or 8 you will end up with an unlicensed ("not genuine") version of Windows 10. Unlicensed versions will still "work" by and large, but you will have reduced functionality. 

Windows has not suddenly become "free", and will still be factored into the retail price of new Windows computers. This is just Microsoft's way of ensuring as many current Windows users as possible will be able to upgrade their home computers for no additional cost. The original free upgrade does not "expire" or become a limited version after a certain time.
There are, though, “edge cases” that add up to a small fraction of Windows users. If you want a fresh copy of Windows 10 to install on a home-built computer or on a virtual machine (like Parallels or VMware Fusion on a Mac), you might have to pay for the operating system.