Following my post on the importance of Flash and other plugins, I must add that one of the things they can do is to add more languages to the client-side of the web. Silverlight adds support to IronPython and IronRuby to the browser and Gestalt makes it really simple and javascript-like.
Another feature in the gestalt project is the HTML5 video player for Internet Explorer, via Silverlight. It might require the installation of a plugin (hey, 99% of the users1 already have at least one installed), but makes the html5 video tag work on the most infamous browser out there. This is an example of using a plugin to extend the capabilities of the browser, even for embracing open standards.
1Or whatever are the stats for Adobe Flash on desktop computers.