Windows 10 Beta — Free Windows 10 for Everyone!
Like the saying goes; "If you are not paying for it, you're not the customer; you're the product being sold" but I'm not sure most even care about that anyway.
So to be part of the programme and to use Windows 10 Beta for free you have to become a Windows Insider, just follow the link, click "Join the Program" and register with your Outlook email (given you have one, if not, make a new one).
This differs from the usual upgrade offer as this is a beta version which users receive early updates and don't have to pay for it, legally.
Mind you, you might think about buying one of those really cheap Windows 10 keys from eBay but as per Microsoft, those are not genuine so you end up getting an invalid/deactivated key as they end up tracking it down.
Windows 10 - Windows Insider Information
All you need to do is read the program agreement and the privacy statement, click that you accept the terms of the program agreement and hit the Submit button. Once you are an Insider, you'll be able to install Windows Insider Previews and start giving feedback directly to Windows engineers. You'll receive a Welcome email and periodic newsletters with updates on the latest preview features, as well as occasional surveys and invites to special events.
- The experimental and early prerelease software and services might not be tested.
- You might experience crashes, security vulnerabilities, data loss, or damage to your device.
- When the software is installed on a mobile device: (i) it is not intended as a substitute for your primary means of telecommunications; (ii) in some circumstances, you may not be able to place calls (including calls for emergency services); (iii) it may inadvertently damage your device rendering it inoperable; and (iv) you may experience increased data charges.
- The Program collects and transmits data to Microsoft regarding activities on your device, including personal information; Microsoft shares some data with partners. See the Privacy Statement for more information.
- You will receive communications about the program and related promotions. Once you’ve joined the program, to stop receiving such communications you must leave the program.