Microsoft Silverlight is a browser plugin that enables interactive websites to be created and viewed. It could be seen as the more advanced rendition of Adobe Flash.
According to the Silverlight webpage, the plugin will be available in the second quarter of 2008 and will be targeted towards Windows Mobile 6.
I am not sure how many current sites use Silverlight technology to power their content, but this is good news. Hopefully, Windows Mobile browsers in the future will be able to utilize this plugin to play back more content.