The Beta of Windows 7 and Windows Server 2008 R2 have been released for TechNet and MSDN subscribers. It is available from here. For non-MSDN and TechNet subscribers, the beta of Windows 7 will be available on Friday 9 January 2009 from here.
Windows 7 and Windows Server 2008 R2 are based on the same code base, and are derived from Windows Vista and Windows Server 2008. Microsoft has built a lot of functionality into the new releases, building on what foundation work was done in Windows Server 2008 and Windows Vista. If an application runs well on Windows Vista, it will most likely run well on Windows 7.
For me, the improvements in Windows 7 are:
Please ensure that you download the beta from one of the Microsoft web sites and not from a 3rd party or peer-to-peer network. You know what you get from Microsoft, from other sources, you may have some extra non-Microsoft code included. The beta Expires 1 August 2009.
For more information, see the Windows Product Group Blog