According to Microsoft Silverlight - Wikipedia, the free encyclopedia
Microsoft Silverlight is a browser plugin that allows web applications to be developed with features like animation, vector graphics, and audio-video playback - features that characterize a rich internet application. Silverlight competes with products such as Adobe Flash, Adobe Flex, Adobe Shockwave, Java FX, and Apple QuickTime. Version 2.0 brought improved interactivity and allows developers to use .NET languages and development tools when authoring Silverlight applications.
I have it installed, but for the life of me can't identify anywhere that uses it except for Microsoft's Silverlight site.... But it's a Beta of a new product, so you honestly can't expect much there. But it'll be nice for the .net programmers to not have to learn Flash, I guess.
Also, there is no 64 bit version. A little fact which I view with a little amusement since MSFT is supposed to be leading the way into consumer based 64 bit computing.
Sorry if I seem a little down on it. I'd be a lot more excited if it could read/run Flash and Flash~like animation on a 64 bit browser. But it doesn't, and so I (we?) remain stuck using 32 bit browsers.