The "Avalon" Community Technology Preview (CTP) which became available to the MSDN members in November of 2004, is now available to the general public in response to numerous requests.
This is what the download page says:
"Avalon" is the code name for Microsoft's unified presentation subsystem for Windows. It consists of a display engine and a managed-code framework. "Avalon" unifies how Windows creates, displays, and manipulates documents, media, and user interface. This enables developers and designers to create visually-stunning, differentiated user experiences that improve customer connection. When it ships, scheduled for 2006, "Avalon" will be available on Windows XP, Windows Server 2003 and all future releases of the Windows operating system. When delivered, "Avalon" will become Microsoft's strategic user interface (UI) technology.
Have you downloaded the tools yet?