This free plug in from Microsoft lets Windows, Mac and Linux users to exhibit animated ads, play quality videos and run games within the web browser. Up till now it was known as Windows Presentation Foundation/Everywhere or WPF/E. This cross browser and cross platform plug in delivers rich media experience. It uses Windows Media Video or WMV to enable compatibility with content available on the internet and support for vide experiences ranging from full screen HD to mobile.

Have a look at the top ten reasons for using Silverlight by Tim Sneath:

It supports playback of WMV files on both PC and Macintosh, with many options for interactivity during playback; with just a couple of lines of code, you can provide a platform-neutral way to handle all your movie files. Silverlight supports full-screen 720p video and offers seamless transitions between full-screen and windowed mode without losing your position in the video (something that media sites are crying out for today).

By separating markup (XAML) from code, Silverlight provides a familiar web metaphor for designers and developers. You can embed XAML directly within an HTML file if you want a simple, monolithic solution, or you can keep the two separate to enforce a delineation between different web development roles.

Silverlight and HTML integrate seamlessly together. Every XAML element can be accessed or manipulated from the same client-side JavaScript that would be used to interact with any DHTML element: there are no artificial boundaries or barriers, and you can even overlay HTML elements on top of Silverlight content (simply by creating a windowless frame). We'll also make it very easy for an ASP.NET AJAX developer to add Silverlight content.

You can embed XAML directly into your HTML pages; there's nothing binary or opaque about the format. There are only three steps necessary to add animation or media to your RIA application: (i) include a standard JavaScript file in your HTML header; (ii) call a function to create the Silverlight object anywhere on the screen; (iii) add some XAML content (an animation, some media) for runtime delivery.

You have full runtime interactivity with Silverlight content. The contents of the XAML file can be completely server-generated, to contain information populated from a database. From JavaScript, it's just a matter of calling the createFromXaml method to add or remove elements dynamically at runtime. There's nothing that you can only create or manipulate at design-time.
Silverlight is just a 1MB download on a PC (slightly more on a Macintosh because the universal package contains both Intel and PowerPC versions); it supports Windows XP and above, with Windows 2000 support to come.

Silverlight is blindingly fast - for example, you can play many videos simultaneously without stuttering or dropping frames (subject to network bandwidth, of course). We're introducing a new video brush in Silverlight that allows you to use video as a texture for any 2D object (a rectangle, an ellipse or a path). This is going to allow designers incredible power to use media in new ways that have never been accessible through other existing technologies.

Silverlight is both client- and server-agnostic. There's no difference between the Macintosh and PC runtimes; you don't need any Microsoft software on the server if you don't want to - you can deliver a great Silverlight experience from an Apache / Linux server to a Mac OS 10.4 client.

Silverlight is almost 100% upward compatible with WPF. Animation, 2D vector graphics, media, text - they're all present in Silverlight and the concepts you've learnt in WPF carry forward (although Silverlight is a subset - it doesn't support WPF features such as 3D, data binding or templates). You can use the same tools (e.g. Expression Design) to generate content for Silverlight; you can take XAML from Silverlight and use it in a WPF application when you want to scale up and take full advantage of your local machine.

I can't reveal this yet - there's a big surprise up our collective corporate sleeve that will be announced at MIX. I hate to hold back on you, but anticipation is part of the pleasure, as my mother used to tell me as a child when I was waiting impatiently for Christmas to come!

Since Silverlight offers better video quality as compared to Flash therefore it can be a threat for it. You can download the plug in and try its sample content here.