The current Windows PowerShell build is Release Candidate 1 Refresh.

It's available for download at the time of writing from here.

To download you need to register. To register you need a Microsoft Passport or Microsoft Live ID. In my experience it's useless attempting to login to Passport using any browser other than Internet Explorer.

I'm not quite sure why Microsoft has insisted on a need for registration for this download, but they have. Since you are getting your hands on a great admin tool free, it's not too big an inconvenience.

You also need the .NET Framework 2.0 installed because PowerShell uses .NET Framework 2.0 classes behind the scenes.

Windows PowerShell runs on Windows Server 2003 (RTM, SP1), Windows Server 2003 R2 and Windows XP (RTM, SP1 and SP2) and Windows Vista.