Microsoft Silverlight is a new cross-browser, cross-platform client technology that helps you design, develop, and deliver media-enabled experiences and rich interactive applications on the Web.

Silverlight appeals to both designers and to developers because it provides a powerful platform that makes it easy to develop Web applications with professional-quality graphics, audio, and video for an engaging user experience. Silverlight also offers powerful tools that will improve the productivity of both designers and developers.

As always the best reference for developers on silverlight can be found in MSDN - 

Microsoft Silverlight portal also has many great resources -

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