Microsoft® Silverlight™ is a cross-browser, cross-platform plug-in for delivering the next generation of .NET based media experiences and rich interactive applications for the Web. Silverlight offers a flexible programming model that supports AJAX, VB, C#, Python, and Ruby, and integrates with existing Web applications. Silverlight supports fast, cost-effective delivery of high-quality video to all major browsers running on the Mac OS or Windows.
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.
Its quite interesting & exciting tool. More on Silverlight can be found at MSDN as usual 
or Silverlight portal.
Hope to dig in more in this tool in the near future.
