View and Edit Live Silverlight Examples
Silverlight 1.0 Unleashed is days away from shipping.  To celebrate, you can (1) pre-order your copy now and (2) check out examples from the book at http://www.adamnathan.net/silverlight/code.

You can not only view the HTML/XAML/JavaScript source and the resulting output, but you can edit all of it on-the-fly. 
There's a screencast that demonstrates it.