Microsoft has released Windows 8.1 Preview today during its BUILD 2013 keynote and you can now download it for free. Windows 8.1 Preview has many new exciting features like a start button and more customization options.
It is available as an online installer which downloads Windows 8.1 Preview from the Windows Store and an ISO version in these languages: Arabic, English (US), Chinese (Simplified), Chinese (Traditional), French, German, Japanese, Korean, Portuguese (Brazil), Russian, Spanish, Swedish, and Turkish. If you want to install from ISO, you can use this product key: NTTX3-RV7VB-T7X7F-WQYYY-9Y92F
Keep in mind that it is a pre-release version and many contain many bugs.