Microsoft has released Windows 8 Developer Preview as free download

The Windows Developer Preview is a pre-beta version of Windows 8 for developers. These downloads include prerelease 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 prerelease software may not appear in the final version. Some product features and functionality may require advanced or additional hardware, or installation of other software.
Windows Developer Preview with developer tools English, 64-bit (x64)

DOWNLOAD (4.8 GB)
Sha 1 hash - 6FE9352FB59F6D0789AF35D1001BD4E4E81E42AF
All of the following come on a disk image file (.iso). See below for installation instructions.
64-bit Windows Developer Preview
Windows SDK for Metro style apps
Microsoft Visual Studio 11 Express for Windows Developer Preview
Microsoft Expression Blend 5 Developer Preview
28 Metro style apps including the BUILD Conference app

Windows Developer Preview English, 64-bit (x64)

DOWNLOAD (3.6 GB)
Sha 1 hash – 79DBF235FD49F5C1C8F8C04E24BDE6E1D04DA1E9
Includes a disk image file (.iso) to install the Windows Developer Preview and Metro style apps on a 64-bit PC.

Windows Developer Preview English, 32-bit (x86)

DOWNLOAD (2.8 GB)
Sha 1 hash - 4E0698BBABE01ED27582C9FC16AD21C4422913CC
Includes a disk image file (.iso) to install the Windows Developer Preview and Metro style apps on a 32-bit PC.

Live Connect

Sign up for the Live Connect technical preview.
Live Connect provides developers a set of controls and APIs that enable applications to integrate Single Sign On (SSO) with Microsoft connected accounts and enable users to access information from SkyDrive, Hotmail, and Messenger.

System Requirements

Windows Developer Preview works great on the same hardware that powers Windows Vista and Windows 7:
1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
16 GB available hard disk space (32-bit) or 20 GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
Taking advantage of touch input requires a screen that supports multi-touch

  Windows 8 Developer Preview Download