Silverlight

Silverlight is a cross platform, cross browser .NET plug-in that enables designers and developers to build rich media experiences and RIAs for browsers


I'd recommend watching these (free) conference sessions from the MIX conference that cover Silverlight 1.1 with .NET scenarios:

Building Silverlight Applications using .NET (Part 1)
Building Silverlight Applications using .NET (Part 2)
Extending the Browser Programming Model with Silverlight