Microsoft is offering a free download of Windows 7 Release Candidate (RC). Use Internet Explorer to download it – http://www.microsoft.com/windows/windows-7/download.aspx
This RC version will expire on June 1, 2010. However, starting on March 1, 2010, your PC will begin shutting down every two hours, effectively making it unusable (?).
Note, while a release candidate is considered a more stable version than a "Beta", this software should be used with caution:
"Information about Windows 7 is preliminary and subject to change. Some product features of Windows 7, such as the ability to watch and record live TV or navigation through the use of "touch," may require advanced or additional hardware. The features and functionality you find in the pre-release product may not appear in the final version of Windows 7. If we change the software before it’s released, we’ll change this information accordingly. We’re not making express or implied warranties with this information."