For those of you who've seen "Monad", you'll want the RC1 download links first:
Windows PowerShell RC1 - x86
Windows PowerShell RC1 - x64
Windows PowerShell RC1 - Documentation Pack
For those of you who haven't seen it yet, Windows PowerShell (formerly "Monad") provides a command-shell environment based on the .net Framework. It includes a new concept of cmdlets (pronounced "commandlets"), which are simple and discrete commands provided by the shell (you can create your own custom cmdlets and add them to the shell), a new language for scripting the shell, and more.
Some folks may wonder...why a new shell, and why a new language? Take a look at the PowerShell User Guide in the documentation pack, particularly at Appendix A, which lists examples of VBScript vs. PS script examples. The degree to which many common use cases have been dramatically simplified is awesome. If you're a command-line geek, you should definitely check this out.
Note that according to the download page, Windows PowerShell is supported on Windows Vista, Windows Server 2003, and Windows XP.
Apparenly there were some issues with the initial download, so a new download, the RC1 Refresh has been posted. If you installed RC1 between April 25th and May 3rd, you should uninstall that version, and install the RC1 Refresh, which is available from the links above.