Silverlight is Microsoft's answer to clever interactive web site technologies such as Flash or Javascript in web browsers, and the latest version is now out. I've not come across that many sites using it yet (apart from Microsoft.com of course) but it's always worth having the latest version of these programs.

It's also worth noting that Silverlight is available in more forms than you might expect for a Microsoft technology. It can be installed for Firefox and Safari as well as Internet Explorer on Mac and Windows. Support for the Opera browser is planned and there is an open source project supported by Microsoft called Mono working to bring Silverlight to Linux.

You can get Silverlight at http://www.microsoft.com/silverlight