Microsoft have just announced Silverlight, the new branding for what was previously known as WPF/E. They've also provided a lot more details about the specifications and their future plans.
Silverlight is the new Adobe (Macromedia) Flash competitor from Microsoft, it is based on the new Windows Presentation Foundation graphics engine found in Vista. It turns out that it will be very tightly integrated with .Net even to the point of shipping with its own cut down runtime.
The download should be around 2MB though initially it will not contain 3D capabilities. IE, Firefox and Safari will be supported at launch.
Silverlight website: http://www.microsoft.com/silverlight/