Adobe Flash is the most popular competitor to Silverlight. Both support browser plug-ins, and enables execution of rich content for the Web. Flash is not a new technology, and already has had a longer life span as compared to Silverlight. But it does not have a huge community as would be expected; it may be because of the limited development tools which are not kwon to most developers. Flash uses ActionScript as the programming language and Flex as the programming environment, which most developers are not familiar with.
For ASP.NET developers, to extend their websites using Flash content is not so simple. They need to learn the development environment as mentioned above, like ActionScript and Flex. Apart from them, there is no way to generate Flash content using server-side .NET code, which means it’s difficult to integrate ASP.NET content and Flash content.
Silverlight aims to give .NET developers a better option for creating rich web content. Silverlight provides a browser plug-in with many similar features to Flash, but one that’s designed from the ground up for .NET. Silverlight natively supports the C# language, and uses a range of .NET concepts. As a result, developers can write client-side code for Silverlight in the same language they use for server-side code (such as C# and VB), and use many of the same abstractions (including streams, controls, collections, generics, and LINQ).
Reference : CodeProject.ccom
Chaitanya Pai 