Silverlight is a cross-browser, cross-platform plug-in for delivering the next generation of media experiences and rich interactive applications (RIAs) for the Web. It was previously known as WPF/E. Finally its good to see that Microsoft has come up with a cool name for a cool product. For those of you who dont know, Silverlight works also on Mac OS.
You can download Silverlight runtime or the SDK itself, the runtime being a very small file of around 2mb. Also do checkout some amazing applications showcased at the bottom of this page built using Silverlight.