Adobe Flash is the most popular competitor to Silverlight both supports browser plug-in and enables execution of rich content for the Web. Flash is not the new technology, which is already having long life span as compared to Silverlight. But it does not have huge community as expected; it may be cause of limited development tools, which are not kwon to most of the developers.  Flash uses ActionScript as programming language and Flex as programming environment, which most of the developer are far from these technologies.

For ASP.NET developers to extend their websites using flash content is not so simples, they need to learning development environment as mentioned above like ActionScript and Flex, apart from that 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).