Avalon Community Technical Preview
"Avalon" is the code name for the presentation subsystem class libraries in WinFX. MSDN Subscribers can download the November 2004 Community Technical Preview. It runs on Windows XP as well as Windows 2k3. It requires .NET 2.0 beta which I already have on my machine for another cool beta: MSH
More information on the MSDN article on Avalon November 2004 Community Technical Preview and the Avalon news groups:
Avalon development topics.
WinFX SDK topics.
General WinFX developer announcements.
Update: Forgot to add XamlPad which allows you to play with XAML after installing Avalon.