The project formerly known as Windows Presentation Foundation everywhere (WPF/e) has been re-named Sliverlight in preparation for its final release. Prior to WPF/e it was known as Jolt. Silverlight is a cross-platform and cross-browser plugin for creating rich internet experiences and applications for the web. It uses a subset of Xaml and WPF. You can read Tim Sneath's top 10 reasons you might be interested in Silverlight here.
