You can now download the release version of Windows 10 directly from Microsoft. The tools on that page allow you to upgrade the computer you’re using, or to create bootable disc or thumb drive images, which can then be used to install Windows 10 from scratch on another computer. Both the Home and Pro versions are available, in 32 and 64 bit form.
If you’re running Windows 7 or 8.x, and you choose to perform an upgrade from the site linked above, you’ll get the Home version if you’re currently running one of the Home variants, and Pro otherwise.
It’s still not completely clear what happens when you don’t have a legitimate Windows 7 or 8.x license. At some point, you’ll be asked to enter a license key, and without one, presumably Windows 10 will stop functioning or suffer from reduced functionality. The same goes for in-place upgrades; as Microsoft has said, if you have a non-valid install of Windows 7 or 8.x and upgrade it to Windows 10, it will continue to be non-valid, with all that entails.
Update: My Windows 10 test computer is running whatever version is being provided via the Windows Insider program. It looks like the final release version, and has the build number Microsoft planned to use for the release: 10240. Because I joined the Windows Insider program (which involved having updates pushed to the computer regularly, and being asked to provide ratings and feedback), I’m now running Windows 10 Pro on a computer that previously ran Windows XP, and it didn’t cost anything, and it’s completely legit. Of course, if I ever want to stop logging in to Windows 10 with my Microsoft ID, I’ll have to purchase a Windows 10 license.