Today at the NAB conference,  Microsoft announces Silverlight, formerly known as WPF/E. Here's the official boilerplate:

Microsoft® 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.

This is a cross-platform effort, so Silverlight runs on both Windows and Mac platforms and inside IE, Firefox and Safari.  

Check out the new site.

Microsoft Silverlight