Today Microsoft is showing preview of Windows 8 at BUILD conference and here is a great news for all people who are eagerly waiting to try their hands on Windows 8.
Microsoft has released Windows 8 Developer Preview build to public which can be downloaded using following link:
The Developer Preview of Windows 8 will expire on March 11th, 2012 so you can enjoy it for approx. 6 months.
According to Microsoft:
Windows 8 Developer Preview is a pre-beta version of Windows 8 for developers. These downloads include pre-release software that may change without notice. The software is provided as is, and you bear the risk of using it. It may not be stable, operate correctly or work the way the final version of the software will. It should not be used in a production environment. The features and functionality in the pre-release software may not appear in the final version. Some product features and functionality may require advanced or additional hardware, or installation of other software.
You can download both 32-bit and 64-bit editions. The download also includes a 64-bit (x64) build with development tools to build apps. The download also includes a suite of sample applications which are merely illustrations of potential apps, not apps that Microsoft intends to ship with Windows 8.
Please note that "Windows 8 Developer Preview with Developer Tools" doesn't support upgrade from Windows XP, Vista or 7. You'll need to do a clean install.
If you want to upgrade existing Windows version to Windows 8, you'll need to use either 32-bit or 64-bit edition of Windows 8 Developer Preview without Developer Tools. One more thing to keep in mind! If you upgrade from Windows Vista or 7, Windows 8 will keep your accounts, files and settings but if you upgrade from Windows XP, Windows 8 will only keep your accounts and files but not settings.
This is a developer preview release and is not meant for production. It is not even a beta release. Microsoft confirms that they'll be updating the release with various quality updates and drivers over the coming weeks / months to test their overall update and telemetry mechanisms.
You can learn more about new features in Windows 8 using official Windows 8 Fact sheet and guide released by Microsoft:
Download Windows 8 Developer Preview Guide (PDF) Download Windows 8 Developer Preview Guide (XPS)
Update: The PDF and XPS guides have been removed by Microsoft.