Silverlight is a compact (4MB) runtime of the .NET framework that is designed to run in IE, Firefox and Safari. No Linux support yet.
The money quote:
Silverlight is excellent technology and those asking why developers and application providers won’t just stick to flash only need to look at XAML, the runtime speed and size and the flexible options with programming languages combined with very strong multimedia support to start to see the answer. Microsoft have a battle on their hands to convince the developer and designer communities that their platform is the best platform, but most of this convincing won’t be a technical showdown but rather the establishment of trust between users and Microsoft as the vendor of this new platform.
