Take a look at this XAML Viewer presented by Bob Bartholomay.
I’ve been messing around with a Silverlight 3 app that allows you to open and view XAML files located on your locale machine. Sure, you can do that already with your favorite editor including Visual Studio but I wanted to show more information easily so the viewer did not have to continually copy and paste or search help and loose the whole picture of the XAML file while they learn about a particular piece of it..