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From the site:

Indigo and Avalon are the codenames for two strategic developer technologies that Microsoft plans to ship in 2006 as part of the Windows "Longhorn" operating system. In addition, Microsoft is making these technologies available on Windows XP and Windows Server 2003. The Indigo and Avalon Beta1 "RC" enables developers to continue experimenting with early builds of these technologies, get acquainted with the development experience, and provide Microsoft with feedback.