So what is Microsoft Silverlight? Well, Microsoft Silverlight is a cross-platform. cross-browser plug-in that will allow you to receive the next generation of .NET based media experiences as well as interactive applications for the Web.
Silverlight is a great and flexible programming model that supports Ruby, Python, C#, VB, AJAX and will work together with existing web applications. Microsoft Silverlight supports, high quality, cost effective, fast video to all the top browsers that run using either the Windows or Mac OS.
There are a few open source communities for Silverlight , but one open source community that sprung up the moment the ASP.NET MVC initially released was MVCContrib who are a vibrant open source. Of course not forgetting the project that did start straight away around the Silverlight 2 initial releases: SilverlightContrib, the project has been doing really well and had just released a set of free open source silverlight controls.
Source: timheuer